Double perovskite

ABSTRACT

The present invention relates to a semiconductor device comprising a semiconducting material, wherein the semiconducting material comprises a compound comprising: (i) one or more first monocations [A]; (ii) one or more second monocations [B I ]; (iii) one or more trications [B III ]; and (iv) one or more halide anions [X]. The invention also relates to a process for producing a semiconductor device comprising said semiconducting material. Also described is a compound comprising: (i) one or more first monocations [A]; (ii) one or more second monocations [B I ] selected from Cu + , Ag +  and Au + ; (iii) one or more trications [B III ]; and (iv) one or more halide anions [X].

FIELD OF THE INVENTION

The present invention relates a semiconductor device comprising asemiconducting material and a process for producing a semiconductordevice. The invention also relates to novel compounds comprising metalmonocations.

The work leading to this invention has received funding from theEuropean Community's Seventh Framework Programme (FP7/2007-2013) undergrant agreement agreements 279881, 239578 (ALIGN), 604032 of the MESOproject and 604391 (Graphene Flagship).

BACKGROUND OF THE INVENTION

The development of efficient, cost-effective, and scalable solar energytechnologies constitutes a top priority in the global energy researchagenda. The past five years have witnessed a revolution in photovoltaicsresearch with the discovery of the organic-inorganic perovskitemethylammonium lead triiodide (MAPbI₃, MA=CH₃NH₃), leading to solarcells with an energy conversion efficiency exceeding 20% (Green et al,The emergence of perovskite solar cells, Nature Photon. 8, 506-514,2014). Despite the unprecedented progress in perovskite photovoltaics,the presence of lead in MAPbI₃ raises concerns about the potentialenvironmental impact of these devices. Numerous experimental andcomputational efforts have been devoted to searching for lead-freealternatives to MAPbI₃. However, to date no material can rival theremarkable optoelectronic properties of MAPbI₃.

MAPbI₃ is an ABX₃ perovskite where the B sites are occupied by the heavymetal cation Pb²⁺, the X sites are occupied by the halide anion I⁻ andthe A sites are occupied by the organic cation MA+. The most obviousroute to replacing Pb in this compound would be via substitution ofother Group 14 elements, for instance Sn and Ge. However, both elementstend to undergo oxidation, for example from Sn²⁺ to Sn⁴⁺, leading to arapid degradation of the corresponding halide perovskites (Stoumpos etal, Semiconducting tin and lead iodide perovskites with organic cations:phase transitions, high mobilities, and near-infrared photoluminescentproperties, Inorg. Chem. 52, 9019-9038, 2013). Another possible avenuetowards lead replacement is substitution by other divalent cationsoutside of Group 14 elements. To check this possibility the inventorshave performed a high-throughput computational screening of potentialcandidates, but did not succeed in identifying compounds matching theremarkable optoelectronic properties of MAPbI₃. Thus, it hasunexpectedly been found that no other divalent metal or metalloidcations have the potential to be used instead of lead.

A structure related to perovskites is that of double perovskites whichhave the formula A₂B′B″X₆. Oxide double perovskites are known (Vasala &Karppinen, A ₂ B′B″O ₆ perovskites: A review, Prog. Solid St. Chem. 43,1-36, 2015). However, halide double perovskites have not beensynthesised. Halide double perovskites using thallium have been proposedin a theoretical study (Giorgi & Yamashita, Alternative, lead-free,hybrid organic-inorganic perovskites for solar applications: A DFTanalysis, Chem. Lett. 44, 826-828, 2015). However, the replacement oflead by thallium only worsens the toxicity problem. It would appear atpresent that the there is no clear way forward in the photoactiveperovskite field to overcome the problems associated with lead.

There is a need to develop new semiconducting materials which do notcomprise lead (or any other toxic heavy metal). Also, compounds which donot comprise lead need to be stable, for instance with respect tooxidation. New semiconducting materials with a range of electronicproperties are also desired, as are new semiconducting materials usefulas photoactive materials.

SUMMARY OF THE INVENTION

The inventors have developed a new family of organic-inorganic halideperovskites, whereby lead is completely replaced by one or moretrications (for instance bismuth or antimony) and one or moremonocations (for instance noble metal cations). Using first-principleselectronic structure calculations, double perovskites such as FA₂AgBiI₆(FA=CH(NH₂)2) have been identified as a stable compounds with a band gapsuitable for solar cell applications and small carrier effective masses,bearing a striking resemblance to MAPbI₃. Compounds of this type havebeen successfully synthesized. More generally, the inventorsinvestigations have revealed the existence of a hitherto unknown familyof materials comprising one or more first monocations [A]; one or moresecond monocations [B^(I)]; one or more trications [B^(III)]; and one ormore halide anions [X], for instance double perovskites of formula[A]₂[B^(I)][B^(III)][X]₆ and associated layered double perovskites.

This new class of materials allows the use of lead (and other toxicheavy metals) to be avoided completely, providing a significantenvironmental benefit. The materials have a strong potential foroptimizing lead-free perovskite photovoltaics. The novel doubleperovskites are also likely to be stable with respect to oxidation.

The invention therefore provides a semiconductor device comprising asemiconducting material, wherein the semiconducting material comprises acompound comprising:

-   -   (i) one or more first monocations [A];    -   (ii) one or more second monocations [B^(I)];    -   (iii) one or more trications [B^(III)]; and    -   (iv) one or more halide anions [X].

The invention also provides a process for producing a semiconductordevice comprising a semiconducting material, wherein the semiconductingmaterial comprises a compound comprising:

-   -   (i) one or more first monocations [A];    -   (ii) one or more second monocations [B^(I)];    -   (iii) one or more trications [B^(III)]; and    -   (iv) one or more halide anions [X],    -   which process comprises:        (a) disposing a second region on a first region, which second        region comprises a layer of said semiconducting material.

The invention further provides a compound comprising:

(i) one or more first monocations [A];(ii) one or more second monocations [B^(I)] selected from Cu⁺, Ag⁺ andAu⁺;(iii) one or more trications [B^(III)]; and(iv) one or more halide anions [X].

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows DFT/LDA calculated electronic band structure of MAPbI₃,along high-symmetry lines in the Brillouin zone.

FIG. 2 shows DFT/LDA calculated electronic band structure of MA₂AgBi₆,along high-symmetry lines in the Brillouin zone.

FIG. 3 shows DFT/LDA calculated electronic band structure of MA₂CuBiI₆,along high-symmetry lines in the Brillouin zone.

FIG. 4 shows DFT/LDA calculated electronic band structure of MA₂AuBiI₆,along high-symmetry lines in the Brillouin zone.

FIG. 5 shows DFT/LDA calculated electronic band structure of FA₂AgBiI₆,along high-symmetry lines in the Brillouin zone.

FIG. 6 shows DFT/LDA calculated electronic band structure of MA₂BiNaI₆,along high-symmetry lines in the Brillouin zone.

FIG. 7 shows the powder XRD diffraction pattern of MA₂AgBiI₆.

FIG. 8 shows the powder XRD diffraction pattern of FA₂AgBiI₆.

FIG. 9 shows the UV-Vis spectrum for MA₂AgBiI₆.

FIG. 10 shows the UV-Vis spectrum for FA₂AgBiI₆.

FIG. 11 shows the UV-Vis spectrum for MA₂AuBiI₆.

FIG. 12 shows photoluminescence spectra of MA₂AgBiI₆ and FA₂AgBiI₆.

FIG. 13 shows the results of characterisation of Cs₂BiAgCl₆: (a) showssingle crystal diffraction patterns along three different planes, 0kl,h0l and hk0; (b) shows the UV-Vis optical absorption spectrum; (c) showsthe steady-state photoluminescence (PL) spectrum; and (d) shows timeresolved photoluminescence decay.

FIG. 14 shows the calculated band structure of the experimentallymeasured crystal structure for Cs₂BiAgCl₆.

FIG. 15 shows the XRPD pattern for Cs₂BiAgBr₆.

FIG. 16 shows the room temperature optical absorption spectra ofCs₂BiAgBr₆.

FIG. 17 shows the room temperature photoluminescence spectrum measuredfor Cs₂BiAgBr₆.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The term “photoactive material”, as used herein, refers to a materialwhich either (i) absorbs light, which may then generate free chargecarriers; or (ii) accepts charge, both electrons and holes, which maysubsequently recombine and emit light. A photoabsorbent material is amaterial which absorbs light, which may then generate free chargecarriers (e.g electrons and holes). Photoactive materials are examplesof semiconducting materials. A “photoemissive material” is a materialwhich absorbs light of energies higher than band gap and reemits lightat energies at the band gap

The term “monocation”, as used herein, refers to any cation with asingle positive charge, i.e. a cation of formula A⁺ where A is anymoiety, for instance a metal atom or an organic moiety. The term“dication”, as used herein, refers to any cation with a double positivecharge, i.e. a cation of formula A²⁺ where A is any moiety, for instancea metal atom or an organic moiety. The term “trication”, as used herein,refers to any cation with a triple positive charge, i.e. a cation offormula A³⁺ where A is any moiety, for instance a metal atom.

The term “semiconductor” or “semiconducting material”, as used herein,refers to a material with electrical conductivity intermediate inmagnitude between that of a conductor and a dielectric. A semiconductormay be an negative (n)-type semiconductor, a positive (p)-typesemiconductor or an intrinsic (i) semiconductor. A semiconductor mayhave a band gap of from 0.5 to 3.5 eV, for instance from 0.5 to 2.5 eVor from 1.0 to 2.0 eV (when measured at 300 K).

The term “n-type region”, as used herein, refers to a region of one ormore electron-transporting (i.e. n-type) materials. Similarly, the term“n-type layer” refers to a layer of an electron-transporting (i.e. ann-type) material. An electron-transporting (i.e. an n-type) materialcould, for instance, be a single electron-transporting compound orelemental material. An electron-transporting compound or elementalmaterial may be undoped or doped with one or more dopant elements.

The term “p-type region”, as used herein, refers to a region of one ormore hole-transporting (i.e. p-type) materials. Similarly, the term“p-type layer” refers to a layer of a hole-transporting (i.e. a p-type)material. A hole-transporting (i.e. a p-type) material could be a singlehole-transporting compound or elemental material, or a mixture of two ormore hole-transporting compounds or elemental materials. Ahole-transporting compound or elemental material may be undoped or dopedwith one or more dopant elements.

The term “alkyl”, as used herein, refers to a linear or branched chainsaturated hydrocarbon radical. An alkyl group may be a C₁₋₂₀ alkylgroup, a C₁₋₁₄ alkyl group, a C₁₋₁₀ alkyl group, a C₁₋₆ alkyl group or aC₁₋₄ alkyl group. Examples of a C₁₋₁₀ alkyl group are methyl, ethyl,propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl or decyl. Examples ofC₁₋₆ alkyl groups are methyl, ethyl, propyl, butyl, pentyl or hexyl.Examples of C₁₋₄ alkyl groups are methyl, ethyl, i-propyl, n-propyl,t-butyl, s-butyl or n-butyl. If the term “alkyl” is used without aprefix specifying the number of carbons anywhere herein, it has from 1to 6 carbons (and this also applies to any other organic group referredto herein).

The term “cycloalkyl”, as used herein, refers to a saturated orpartially unsaturated cyclic hydrocarbon radical. A cycloalkyl group maybe a C₃₋₁₀ cycloalkyl group, a C₃₋₈ cycloalkyl group or a C₃₋₆cycloalkyl group. Examples of a C₃₋₈ cycloalkyl group includecyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclohexenyl,cyclohex-1,3-dienyl, cycloheptyl and cyclooctyl. Examples of a C₃₋₆cycloalkyl group include cyclopropyl, cyclobutyl, cyclopentyl, andcyclohexyl.

The term “alkenyl”, as used herein, refers to a linear or branched chainhydrocarbon radical comprising one or more double bonds. An alkenylgroup may be a C₂₋₂₀ alkenyl group, a C₂₋₁₄ alkenyl group, a C₂₋₁₀alkenyl group, a C₂₋₆ alkenyl group or a C₂₋₄ alkenyl group. Examples ofa C₂₋₁₀ alkenyl group are ethenyl (vinyl), propenyl, butenyl, pentenyl,hexenyl, heptenyl, octenyl, nonenyl or decenyl. Examples of C₂₋₆ alkenylgroups are ethenyl, propenyl, butenyl, pentenyl or hexenyl. Examples ofC₂₋₄ alkenyl groups are ethenyl, i-propenyl, n-propenyl, s-butenyl orn-butenyl. Alkenyl groups typically comprise one or two double bonds.

The term “alkynyl”, as used herein, refers to a linear or branched chainhydrocarbon radical comprising one or more triple bonds. An alkynylgroup may be a C₂₋₂₀ alkynyl group, a C₂₋₁₄ alkynyl group, a C₂₋₁₀alkynyl group, a C₂₋₆ alkynyl group or a C₂₋₄ alkynyl group. Examples ofa C₂₋₁₀ alkynyl group are ethynyl, propynyl, butynyl, pentynyl, hexynyl,heptynyl, octynyl, nonynyl or decynyl. Examples of C₁₋₆ alkynyl groupsare ethynyl, propynyl, butynyl, pentynyl or hexynyl. Alkynyl groupstypically comprise one or two triple bonds.

The term “aryl”, as used herein, refers to a monocyclic, bicyclic orpolycyclic aromatic ring which contains from 6 to 14 carbon atoms,typically from 6 to 10 carbon atoms, in the ring portion. Examplesinclude phenyl, naphthyl, indenyl, indanyl, anthrecenyl and pyrenylgroups. The term “aryl group”, as used herein, includes heteroarylgroups. The term “heteroaryl”, as used herein, refers to monocyclic orbicyclic heteroaromatic rings which typically contains from six to tenatoms in the ring portion including one or more heteroatoms. Aheteroaryl group is generally a 5- or 6-membered ring, containing atleast one heteroatom selected from O, S, N, P, Se and Si. It maycontain, for example, one, two or three heteroatoms. Examples ofheteroaryl groups include pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl,furanyl, thienyl, pyrazolidinyl, pyrrolyl, oxazolyl, oxadiazolyl,isoxazolyl, thiadiazolyl, thiazolyl, isothiazolyl, imidazolyl,pyrazolyl, quinolyl and isoquinolyl.

The term “substituted”, as used herein in the context of substitutedorganic groups, refers to an organic group which bears one or moresubstituents selected from C₁₋₁₀ alkyl, aryl (as defined herein), cyano,amino, nitro, C₁₋₁₀ alkylamino, di(C₁₋₁₀)alkylamino, arylamino,diarylamino, aryl(C₁₋₁₀)alkylamino, amido, acylamido, hydroxy, oxo,halo, carboxy, ester, acyl, acyloxy, C₁₋₁₀ alkoxy, aryloxy,halo(C₁₋₁₀)alkyl, sulfonic acid, thiol, C₁₋₁₀ alkylthio, arylthio,sulfonyl, phosphoric acid, phosphate ester, phosphonic acid andphosphonate ester. Examples of substituted alkyl groups includehaloalkyl, perhaloalkyl, hydroxyalkyl, aminoalkyl, alkoxyalkyl andalkaryl groups. When a group is substituted, it may bear 1, 2 or 3substituents. For instance, a substituted group may have 1 or 2substitutents.

The term “porous”, as used herein, refers to a material within whichpores are arranged. Thus, for instance, in a porous scaffold materialthe pores are volumes within the scaffold where there is no scaffoldmaterial. The individual pores may be the same size or different sizes.The size of the pores is defined as the “pore size”. The limiting sizeof a pore, for most phenomena in which porous solids are involved, isthat of its smallest dimension which, in the absence of any furtherprecision, is referred to as the width of the pore (i.e. the width of aslit-shaped pore, the diameter of a cylindrical or spherical pore,etc.). To avoid a misleading change in scale when comparing cylindricaland slit-shaped pores, one should use the diameter of a cylindrical pore(rather than its length) as its “pore-width” (J. Rouquerol et al.,“Recommendations for the Characterization of Porous Solids”, Pure &Appl. Chem., Vol. 66, No. 8, pp. 1739-1758, 1994). The followingdistinctions and definitions were adopted in previous IUPAC documents(K. S. W. Sing, et al, Pure and Appl. Chem., vol. 57, n04, pp 603-919,1985; and IUPAC “Manual on Catalyst Characterization”, J. Haber, Pureand Appl. Chem., vol. 63, pp. 1227-1246, 1991): micropores have widths(i.e. pore sizes) smaller than 2 nm; Mesopores have widths (i.e. poresizes) of from 2 nm to 50 nm; and Macropores have widths (i.e. poresizes) of greater than 50 nm. In addition, nanopores may be consideredto have widths (i.e. pore sizes) of less than 1 nm.

Pores in a material may include “closed” pores as well as open pores. Aclosed pore is a pore in a material which is a non-connected cavity,i.e. a pore which is isolated within the material and not connected toany other pore and which cannot therefore be accessed by a fluid (e.g. aliquid, such as a solution) to which the material is exposed. An “openpore” on the other hand, would be accessible by such a fluid. Theconcepts of open and closed porosity are discussed in detail in J.Rouquerol et al., “Recommendations for the Characterization of PorousSolids”, Pure & Appl. Chem., Vol. 66, No. 8, pp. 1739-1758, 1994.

Open porosity, therefore, refers to the fraction of the total volume ofthe porous material in which fluid flow could effectively take place. Ittherefore excludes closed pores. The term “open porosity” isinterchangeable with the terms “connected porosity” and “effectiveporosity”, and in the art is commonly reduced simply to “porosity”.

The term “without open porosity”, as used herein, therefore refers to amaterial with no effective open porosity. Thus, a material without openporosity typically has no macropores and no mesopores. A materialwithout open porosity may comprise micropores and nanopores, however.Such micropores and nanopores are typically too small to have a negativeeffect on a material for which low porosity is desired.

The term “compact layer”, as used herein, refers to a layer withoutmesoporosity or macroporosity. A compact layer may sometimes havemicroporosity or nanoporosity.

The term “semiconductor device”, as used herein, refers to a devicecomprising a functional component which comprises a semiconductormaterial. This term may be understood to be synonymous with the term“semiconducting device”. Examples of semiconductor devices include aphotovoltaic device, a solar cell, a photo detector, a photodiode, aphotosensor, a chromogenic device, a transistor, a light-sensitivetransistor, a phototransistor, a solid state triode, a battery, abattery electrode, a capacitor, a super-capacitor, a light-emittingdevice and a light-emitting diode. The term “optoelectronic device”, asused herein, refers to devices which source, control, detect or emitlight. Light is understood to include any electromagnetic radiation.Examples of optoelectronic devices include photovoltaic devices,photodiodes (including solar cells), phototransistors, photomultipliers,photoresistors, light emitting devices, light emitting diodes and chargeinjection lasers.

The term “consisting essentially of” refers to a composition comprisingthe components of which it consists essentially as well as othercomponents, provided that the other components do not materially affectthe essential characteristics of the composition. Typically, acomposition consisting essentially of certain components will comprisegreater than or equal to 95 wt % of those components or greater than orequal to 99 wt % of those components.

Semiconductor Device

The invention provides a semiconductor device comprising asemiconducting material, wherein the semiconducting material comprises acompound comprising:

-   -   (i) one or more first monocations [A];    -   (ii) one or more second monocations [B^(I)];    -   (iii) one or more trications [B^(III)]; and    -   (iv) one or more halide anions [X].

For instance, [A] may be one, two, three or four different firstmonocations. [B^(I)] may be one, two, three or four different secondmonocations. [B^(III)] may be one, two, three or four differenttrications. [X] may be one, two, three or four different halide anions.Typically, [A] is one or two different first monocations. Often, [A] isone first monocation, A. Typically, [B^(I)] is one or two differentsecond monocations. Often, [B^(I)] is one second monocation. Typically,[B^(III)] is one or two different trications. Often, [B^(III)] is onetrication. Typically, [X] is one or two different halide anions. Often,[X] is one halide anion.

For instance, if [A] is one first monocation (A), [B^(I)] is one secondmonocation (B′), [B^(III)] is one trications (B^(III)) and [X] is twohalide anions (X¹ and X²), the crystalline material may comprise acompound of formula AaB^(I) _(bI)B^(III) _(bIII)(X¹,X²)_(c), where a,bI, bIII and c are integers from 1 to 10. If [A], [B^(I)], [B^(III)] or[X] is more than one ion, those ions may be present in any proportion.For instance, AaB^(I) _(bI)B^(III) _(bIII)(X¹,X²)_(c) includes allcompounds of formula A_(a)B^(I) _(bI)B^(III) _(bIII)X¹ _(yc)X² _((1-y)c)wherein y is between 0 and 1, for instance from 0.05 to 0.95. Suchmaterials may be referred to as mixed ion materials (for instance amixed halide material). In such mixed ion materials, the two ions whichare mixed (e.g. X¹ and X²) may be distributed across the sites for thoseions in an ordered or disordered manner.

The one or more first monocations [A] are typically selected from metalmonocations, metalloid monocations and organic monocations, moretypically metal monocations and organic monocations. Metals aretypically metals selected from Groups 1 to 15 of the periodic table, andinclude the alkali metals, the alkali earth metals, the d-blockelements, and p-block metals such as Al, Ga, In, Tl, Sn, Pb and Bi.Metalloids are usually taken to be the elements which are B, Si, Ge, As,Sb and Te. Organic monocations are typically monocations comprising atleast one carbon atom and at least one hydrogen atom. Often, organicmonocations comprise a hydrogen atom bonded to a carbon atom (forinstance in methylammonium, CH₃NH₃ ⁺), but also in some cases do not(for instance in guanidinium, C(NH₂)₃ ⁺). Typically, if [A] comprises anorganic monocations, the organic monocation comprises at least onecarbon atom, at least one hydrogen atom and at least one nitrogencation. For instance, [A] may comprise one or more organic ammoniumcations.

Typically, the one or more first monocations [A] are selected from K⁺,Rb⁺, Cs⁺, (NR¹R²R³R⁴)⁺, (R¹R²N═CR³R⁴)⁺, (R¹R²N—C(R⁵)═NR³R⁴)⁺ and(R¹R²N—C(NR⁵R⁶)═NR³R⁴)⁺, wherein each of R¹, R², R³, R⁴, R⁵ and R⁶ isindependently H, a substituted or unsubstituted C₁₋₂₀ alkyl group, asubstituted or unsubstituted C₂₋₂₀ alkenyl group, a substituted orunsubstituted C₂₋₂₀ alkynyl group, a substituted or unsubstituted C₃₋₂₀cycloalkyl group or a substituted or unsubstituted aryl group. Oftensuch groups are unsubstituted.

Each of R¹, R², R³, R⁴, R⁵ and R⁶ may independently be H, a substitutedor unsubstituted C₁₋₂₀ alkyl group or a substituted or unsubstitutedaryl group, for instance H, an unsubstituted C₁₋₂₀ alkyl group, a C₁₋₂₀alkyl group substituted with one or two aryl groups, an unsubstitutedaryl group, or an aryl group substituted with one or two C₁₋₂₀ alkylgroups. Preferably, Each of R¹, R², R³, R⁴, R⁵ and R⁶ is independentlyH, methyl, hydroxymethyl, ethyl, n-propyl, isopropyl, phenyl,methylphenyl (-Ph-CH₃), ethylphenyl (-Ph-CH₂CH₃), benzyl (—CH₂-Ph) orphenylethyl (—CH₂CH₂-Ph).

Preferably, the one or more first monocations [A] are selected from(NH₄)⁺, (CH₃NH₃)⁺, (HOCH₂NH₃)⁺, (CH₃CH₂NH₃)⁺, (H₂N—C(H)═NH₂)⁺ and(H₂N—C(NH₂)═NH₂)⁺. More preferably the one or more first monocations [A]are methyl ammonium (CH₃NH₃)⁺ or formamidinium (H₂N—C(H)═NH₂)⁺.Alternatively, the one or more first monocations [A] may be Cs⁺.

The one or more second monocations [B^(I)] are typically selected frommetal and metalloid monocations. Preferably, the one or more secondmonocations [B^(I)] are selected from Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Cu⁺, Ag⁺,Au⁺ and Hg⁺. More preferably, the one or more second monocations [B^(I)]are selected from Cu⁺, Ag⁺ and Au⁺. Most preferably, the one or moresecond monocations [B^(I)] are selected from Ag⁺ and Au⁺. For instance,[B^(I)] may be one second monocation which is Ag⁺ or [B^(I)] may be onesecond monocation which is Au⁺.

The one or more trications [B^(III)] are typically selected from metaland metalloid trications. preferably, the one or more trications[B^(III)] are selected from Bi³⁺, Sb³⁺, Cr³⁺, Fe³⁺, Co³⁺, Ga³⁺, As³⁺,Ru³⁺, Rh³⁺, In³⁺, Ir³⁺ and Au³⁺. More preferably, the one or moretrications [B^(III)] are selected from Bi³⁺ and Sb³⁺. For instance,[B^(III)] may be one trication which is Bi³⁺ or [B^(III)] may be onesecond monocation which is Sb³⁺. Bismuth has relatively low toxicitycompared with heavy metals such as lead.

In some embodiments, the one or more second monocations [B^(I)] areselected from Cu⁺, Ag⁺ and Au⁺ and the one or more trications [B^(III)]are selected from Bi³⁺ and Sb³⁺.

Typically, the one or more halide anions [X] are selected from I⁻, Br⁻,Cl⁻ and F⁻. Preferably, the one or more halide anions [X] are selectedfrom Cl⁻, I⁻ and Br⁻. More preferably, the one or more halide anions [X]are selected from I⁻ and Br⁻. In some cases, the one or more halideanions are one halide anion, I⁻.

The compound is typically a crystalline compound. A crystalline compoundis a compound having an extended 3D crystal structure. A crystallinecompound is typically in the form of crystals or, in the case of apolycrystalline compound, crystallites (i.e. a plurality of crystalshaving particle sizes of less than or equal to 1 μm). The crystalstogether often form a layer. An extended 3D crystal structure is astructure wherein the compound comprises an ordered array of ionsoccupying positions within a crystal lattice. The compound typicallyadopts a structure related to the perovskite structure, and inparticular a double perovskite structure or a layered double perovskitestructure. The term “perovskite”, as used herein, refers to a materialwith a three-dimensional crystal structure related to that of CaTiO₃ ora material comprising a layer of material, which layer has a structurerelated to that of CaTiO₃. The structure of CaTiO₃ can be represented bythe formula ABX₃, wherein A and B are cations of different sizes and Xis an anion. In the unit cell, the A cations are at (0,0,0), the Bcations are at (½, ½, ½) and the X anions are at (½, ½, 0). In a doubleperovskite structure, the compound is represented by the formula AB^(I)_(0.5)B^(III) _(0.5)X₃ (equivalent to A₂B^(I)B^(III)X₆) and half of theB sites at (½, ½, ½) are occupied by B^(I) monocations and the otherhalf of the B sites are occupied by B^(III) trications. The occupationof the B sites by the B^(I) and B^(III) cations may be ordered ordisordered. Typically, the arrangement of the B^(I) and B^(III) cationsin the B sites on the lattice are ordered. Often, the B^(I) and B^(III)cations in the B sites on the lattice are arranged in a “chequerboard”fashion with B^(I) and B^(III) cations alternating along each of thethree axes of the lattice. A layered double perovskite has a structurecomprising layers of the double perovskite structure and may, forinstance have the formula A₂B^(I) _(0.5)B^(III) _(0.5)X₄ (equivalent toA₄BB^(III)X₈).

Typically, the compound is a double perovskite compound of formula (I):

[A]₂[B^(I)][B^(III)][X]₆  (I);

wherein: [A] is the one or more first monocations; [B^(I)] is the one ormore second monocations; [B^(III)] is the one or more trications; and[X] is the one or more halide anions.

Often, the one or more first monocations [A] is one first monocation A;the one or more second monocations [B^(I)] is one second monocationB^(I); and the one or more trications [B^(III)] is one tricationsB^(III). The one or more halide anions [X] may be one halide anion X, ortwo or more halide anions [X]. In the latter case (two or more differenthalide anions), the double perovskite is a mixed-halide doubleperovskite.

Typically, the compound is a double perovskite compound of formula (Ia):

A₂B^(I)B^(III)[X]₆  (Ia);

wherein: A is one first monocation; B^(I) is one second monocation;B^(III) is one trication; and [X] is the one or more halide anions, forinstance two or more halide anions.

If [X] is two or more halide anions, the compound may be a mixed halidedouble perovskite compound of formula (Ib):

A₂B^(I)B^(III)X_(6(1-x))X′_(6x)  (Ib);

wherein: A is one first monocation; B^(I) is one second monocation;B^(III) is one trication; X is one halide anion; X′ is a differenthalide anion; and x is from 0.01 to 0.99, for instance from 0.05 to 0.95or from 0.2 to 0.8.

Examples of mixed halide double perovskites according to the inventioninclude (H₂N—C(H)═NH₂)₂AgBiX_(6(1-x))X′_(6x),(H₂N—C(H)═NH₂)₂AuBiX_(6(1-x))X′_(6x),(H₂N—C(H)═NH₂)₂AgSbX_(6(1-x))X′_(6x),(H₂N—C(H)═NH₂)₂AuSbX_(6(1-x))X′_(6x), (CH₃NH₃)₂AgBiX_(6(1-x))X′_(6x),(CH₃NH₃)₂AuBiX_(6(1-x))X′_(6x), (CH₃NH₃)₂AgSbX_(6(1-x))X′_(6x) and(CH₃NH₃)₂AuSbX_(6(1-x))X′_(6x) wherein X is a first halide anion (forinstance I⁻) and X′ is a second, different, halide anion (for instanceBr⁻ or Cl⁻) and x is from 0.01 to 0.99, for instance from 0.05 to 0.95or from 0.2 to 0.8.

Often, however, the compound is a single halide double perovskitecompound of formula (Ic):

A₂B^(I)B^(III)X₆  (Ic);

wherein: A is one first monocation; B^(I) is one second monocation;B^(III) is one trication; and X is one halide anion.

The compound is typically (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂CuBiI₆, (H₂N—C(H)═NH₂)₂AgSbI₆, (H₂N—C(H)═NH₂)₂AuSbI₆,(H₂N—C(H)═NH₂)₂CuSbI₆, (H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆,(H₂N—C(H)═NH₂)₂CuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(H₂N—C(H)═NH₂)₂CuSbBr₆, (H₂N—C(H)═NH₂)₂AgBiCl₆, (H₂N—C(H)═NH₂)₂AuBiCl₆,(H₂N—C(H)═NH₂)₂CuBiCl₆, (H₂N—C(H)═NH₂)₂AgSbCl₆, (H₂N—C(H)═NH₂)₂AuSbCl₆,(H₂N—C(H)═NH₂)₂CuSbCl₆, (H₂N—C(H)═NH₂)₂AgBiF₆, (H₂N—C(H)═NH₂)₂AuBiF₆,(H₂N—C(H)═NH₂)₂CuBiF₆, (H₂N—C(H)═NH₂)₂AgSbF₆, (H₂N—C(H)═NH₂)₂AuSbF₆,(H₂N—C(H)═NH₂)₂CuSbF₆, (CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆,(CH₃NH₃)₂CuBiI₆, (CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂CuSbI₆,(CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆, (CH₃NH₃)₂CuBiBr₆, (CH₃NH₃)₂AgSbBr₆,(CH₃NH₃)₂AuSbBr₆, (CH₃NH₃)₂CuSbBr₆, (CH₃NH₃)₂AgBiCl₆, (CH₃NH₃)₂AuBiCl₆,(CH₃NH₃)₂CuBiCl₆, (CH₃NH₃)₂AgSbCl₆, (CH₃NH₃)₂AuSbCl₆, (CH₃NH₃)₂CuSbCl₆,(CH₃NH₃)₂AgBiF₆, (CH₃NH₃)₂AuBiF₆, (CH₃NH₃)₂CuBiF₆, (CH₃NH₃)₂AgSbF₆,(CH₃NH₃)₂AuSbF₆, (CH₃NH₃)₂CuSbF₆, Cs₂AgBiI₆, Cs₂AuBiI₆, Cs₂CuBiI₆,Cs₂AgSbI₆, Cs₂AuSbI₆, Cs₂CuSbI₆, Cs₂AgBiBr₆, Cs₂AuBiBr₆, Cs₂CuBiBr₆,Cs₂AgSbBr₆, Cs₂AuSbBr₆, Cs₂CuSbBr₆, Cs₂AgBiCl₆, Cs₂AuBiCl₆, Cs₂CuBiCl₆,Cs₂AgSbCl₆, Cs₂AuSbCl₆, Cs₂CuSbI₆, Cs₂AgBiF₆, Cs₂AuBiF₆, Cs₂CuBiF₆,Cs₂AgSbF₆, Cs₂AuSbF₆ or Cs₂CuSbF₆. It should be noted that any of thesecompounds may be represented with all the subscript indices halved, i.e.(H₂N—C(H)═NH₂)₂AgBiI₆ may equivalently be written(H₂N—C(H)═NH₂)Ag_(0.5)Bi_(0.5)I₃ and (CH₃NH₃)₂AgBiI₆ may equivalently bewritten (CH₃NH₃)Ag_(0.5)Bi_(0.5)I₃.

The first monocation [A] may be a guanidinium cation and the compoundmay therefore be (H₂N—C(NH₂)═NH₂)₂AgBiI₆, (H₂N—C(NH₂)═NH₂)₂AuBiI₆,(H₂N—C(NH₂)═NH₂)₂CuBiI₆, (H₂N—C(NH₂)═NH₂)₂AgSbI₆,(H₂N—C(NH₂)═NH₂)₂AuSbI₆, (H₂N—C(NH₂)═NH₂)₂CuSbI₆,(H₂N—C(NH₂)═NH₂)₂AgBiBr₆, (H₂N—C(NH₂)═NH₂)₂AuBiBr₆,(H₂N—C(NH₂)═NH₂)₂CuBiBr₆, (H₂N—C(NH₂)═NH₂)₂AgSbBr₆,(H₂N—C(NH₂)═NH₂)₂AuSbBr₆, (H₂N—C(NH₂)═NH₂)₂CuSbBr₆,(H₂N—C(NH₂)═NH₂)₂AgBiCl₆, (H₂N—C(NH₂)═NH₂)₂AuBiCl₆,(H₂N—C(NH₂)═NH₂)₂CuBiCl₆, (H₂N—C(NH₂)═NH₂)₂AgSbCl₆,(H₂N—C(NH₂)═NH₂)₂AuSbCl₆, (H₂N—C(NH₂)═NH₂)₂CuSbCl₆,(H₂N—C(NH₂)═NH₂)₂AgBiF₆, (H₂N—C(NH₂)═NH₂)₂AuBiF₆,(H₂N—C(NH₂)═NH₂)₂CuBiF₆, (H₂N—C(NH₂)═NH₂)₂AgSbF₆,(H₂N—C(NH₂)═NH₂)₂AuSbF₆ or (H₂N—C(NH₂)═NH₂)₂CuSbF₆.

The compound is preferably (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbI₆,(H₂N—C(H)═NH₂)₂AuSbI₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆, (CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆,(CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂AgSbBr₆ or (CH₃NH₃)₂AuSbBr₆.Alternatively, the compound may be Cs₂AgBiCl₆, Cs₂AgSbCl₆, Cs₂AuBiCl₆,or Cs₂AuSbCl₆, for instance Cs₂AgBiCl₆.

More preferably the compound is (H₂N—C(H)═NH₂)₂AgBiI₆,(H₂N—C(H)═NH₂)₂AuBiI₆, (CH₃NH₃)₂AgBiI₆ or (CH₃NH₃)₂AuBiI₆. In someembodiments the compound is (H₂N—C(H)═NH₂)₂AgBiI₆ or (CH₃NH₃)₂AgBiI₆.The compound may for instance be (H₂N—C(H)═NH₂)₂AgBiI₆. These compoundscan be particularly effective semiconducting materials, for instance foruse as photoactive materials or photoemissive materials in photovoltaicdevices.

For instance, the compound may be a layered double perovskite compoundof formula (II):

[A]₄[B^(I)][B^(III)][X]₈  (II);

wherein: [A] is the one or more first monocations; [B^(I)] is the one ormore second monocations; [B^(III)] is the one or more trications; and[X] is the one or more halide anions.

Typically, the layered double perovskite compound is a double perovskitecompound of formula (IIa):

A₄B^(I)B^(III)[X]₈  (IIa);

wherein: A is one first monocation; B^(I) is one second monocation;B^(III) is one trication; and [X] is the one or more halide anions, forinstance two or more halide anions.

If [X] is two or more halide anions, the compound may be a mixed halidelayered double perovskite compound of formula (IIb):

A₄B^(I)B^(III)X_(8(1-x))X′_(8x)  (IIb);

wherein: A is one first monocation; B^(I) is one second monocation;B^(III) is one trication; X is one halide anion; X′ is a differenthalide anion; and x is from 0.01 to 0.99, for instance from 0.05 to 0.95or from 0.2 to 0.8.

For instance the compound may be a layered double perovskite of formula(R¹NH₃)₄AgBiI₈, (R¹NH₃)₄AuBiI₈, (R¹NH₃)₄CuBiI₈, (R¹NH₃)₄AgSbI₈,(R¹NH₃)₄AuSbI₈, (R¹NH₃)₄CuSbI₈, (R¹NH₃)₄AgBiBr₈, (R¹NH₃)₄AuBiBr₈,(R¹NH₃)₄CuBiBr₈, (R¹NH₃)₄AgSbBr₈, (R¹NH₃)₄AuSbBr₈, (R¹NH₃)₄CuSbBr₈,(R¹NH₃)₄AgBiCl₈, (R¹NH₃)₄AuBiCl₈, (R¹NH₃)₄CuBiCl₈, (R¹NH₃)₄AgSbCl₈,(R¹NH₃)₄AuSbCl₈, (R¹NH₃)₄CuSbCl₈, (R¹NH₃)₄AgBiF₈, (R¹NH₃)₄AuBiF₈,(R¹NH₃)₄CuBiF₈, (R¹NH₃)₄AgSbF₈, (R¹NH₃)₄AuSbF₈ or (R¹NH₃)₄CuSbF₈,wherein R¹ is an unsubstituted C₃₋₁₂ alkyl group. For instance, R¹ maybe pentyl, hexyl, heptyl or octyl.

The compound used in the semiconducting device of the invention may be ahybrid compound of a perovskite and a double perovskite. Thus, in somecases the compound further comprises one or more metal or metalloiddications [B^(II)]. Preferably, the one or more metal or metalloiddications are selected from Sn²⁺, Pb²⁺, Cu²⁺, Ge²⁺ and Ni²⁺. Morepreferably, the one or more metal or metalloid dications are Sn²⁺ orPb²⁺. The metal or metalloid dications occupy some of the B sites in theperovskite structure meaning that B sites are occupied by trications,dications and monocations.

Thus, the semiconducting material may comprise a compound which is ahybrid double perovskite compound of formula (Iz) or a layered hybriddouble perovskite compound of formula (IIz):

[A]₂[B^(I)]_((2-x)/2)[B^(II)]_(x)[B^(III)]_((2-x)/2)[X]₆  (Iz);

[A]₄[B^(I)]_((2-x)/2)[B^(II)]_(x)[B^(III)]_((2-x)/2)[X]₈  (IIz);

wherein [A] is the one or more first monocations; [B^(I)] is the one ormore second monocations; [B^(II)] is the one or more metal or metalloiddications; [B^(III)] is the one or more trications; [X] is the one ormore halide anions; and x is from 0.0 to 1.98. x may be from 0.2 to 1.8,for instance from 0.5 to 1.5. Often, x is approximately 1.0. Forexample, the compound may be a hybrid double perovskite compound offormula (Iza) or a layered hybrid double perovskite compound of formula(IIza):

[A]₂[B^(I)]_(0.5)Pb[B^(III)]_(0.5)[X]₆  (Iza);

[A]₄[B^(I)]_(0.5)Pb[B^(III)]_(0.5)[X]₈  (IIza);

wherein [A] is the one or more first monocations; [B^(I)] is the one ormore second monocations; [B^(III)] is the one or more trications; and[X] is the one or more halide anions.

The semiconducting material may comprise one or more secondary compoundsin addition to the compound described above (e.g. in addition to thedouble halide perovskite of formula (I)). Examples of secondarycompounds which the semiconducting material may further comprise includea perovskite of formula [J][K][L]₃ wherein: [J] is at least onemonocation (which may be as defined herein for the first monocation A);[K] is at least one metal or metalloid dication (which may for instancebe selected from Ca²⁺, Sr²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Fe²⁺, Co²⁺, Pd²⁺,Ge²⁺, Sn²⁺, Pb²⁺, Yb²⁺ and Eu²⁺); and [L] is at least one halide anion;or a hexahalometallate of formula [J]₂[M][L]₆ wherein: [A] is at leastone monocation (which may be as defined herein for the first monocationA); [M] is at least one metal or metalloid tetracation (which may forinstance be selected from Pd⁴⁺, W⁴⁺, Re⁴⁺, Os⁴⁺, Ir⁴⁺, Pt⁴⁺, Sn⁴⁺, Pb⁴⁺,Ge⁴⁺, and Te⁴⁺); and [L] is at least one halide anion. For instance, thesecondary compound may be MAPbI₃.

The semiconductor device may be an optoelectronic device (for instancephotovoltaic device, a solar cell, a photo detector, a photomultiplier,a photoresistor, a charge injection laser, a photodiode, a photosensor,a chromogenic device, a light-sensitive transistor, a phototransistor, alight-emitting device or a light-emitting diode), a transistor, a solidstate triode, a battery, a battery electrode, a capacitor or asuper-capacitor.

Typically, the semiconductor device is an optoelectronic device.Preferably, the semiconductor device is a photovoltaic device, alight-emitting device (for instance a light emitting diode) or aphotodetector. Most preferably, the semiconductor device is aphotovoltaic device. When the semiconductor device is an optoelectronicdevice, the semiconducting material is typically a photoactive material.In some cases, the semiconducting material is a photoemissive material.

The semiconducting material may comprise greater than or equal to 50 wt% of the compound (e.g. the double perovskite compound). Thesemiconducting material may comprise additional components. Inparticular, the semiconducting material may comprise one or more dopantcompounds. Typically, the semiconducting material comprises greater thanor equal to 80 wt % of the compound (e.g. a double perovskite compoundas defined herein). Preferably, the semiconducting material comprisesgreater than or equal to 95 wt % of the compound as defined herein (e.g.a double perovskite compound), for instance greater than or equal to 99wt % of the compound as defined herein (e.g. a double perovskitecompound as defined herein). The semiconducting material may consist, orconsist essentially, of the compound.

The semiconducting material is typically solid. Typically, thesemiconducting material comprises crystalline material. Thesemiconducting material may be crystalline or polycrystalline. Forinstance the semiconducting material may comprise a plurality ofcrystallites of the compound.

The semiconducting material may be in any form. Typically thesemiconducting material is in the form of a layer, for instance aphotoactive, photoemissive or photoabsorbent, material in the form of alayer. The semiconducting material typically comprises a layer of thecompound. The semiconducting material may consist essentially of a layerof the compound, for instance a layer of a double perovskite compound asdefined herein. The semiconductor device may comprise a layer of saidsemiconducting material (for instance a photoactive material) having athickness of greater than or equal to 50 nm, or having a thickness ofgreater than or equal to 100 nm.

Typically, the semiconductor device comprises a layer of thesemiconducting material, which layer preferably has a thickness of from5 nm to 1000 nm. Preferably, the layer of the semiconducting materialhas a thickness of from 100 nm to 700 nm, for instance from 200 nm to500 nm. The layer of the semiconducting material may consist, or consistessentially of a layer of the compound having a thickness of from 100 nmto 700 nm. For instance, the semiconductor device may comprise a layerof said semiconducting material, which semiconducting material comprisesa double perovskite compound as defined herein, which layer has athickness of greater than or equal to 100 nm. In some devices, the layermay be a thin sensitising layer, for instance having a thickness of from5 nm to 50 nm. In devices wherein the layer of said semiconductingmaterial forms a planar heterojunction with an n-type or p-type region,the layer of said photoactive material may have a thickness of greaterthan or equal to 100 nm. Preferably, the layer of said photoactivematerial has a thickness of from 100 nm to 700 nm, for instance from 200nm to 500 nm. The term “planar heterojunction”, as used herein, meansthat surface defining junction between the semiconducting material andthe n- or p-type region is substantially planar and has a low roughness,for instance a root mean squared roughness of less than 20 nm over anarea of 25 nm by 25 nm, for instance a root mean squared roughness ofless than 10 nm, or less than 5 nm, over an area of 25 nm by 25 nm.

The semiconducting material often acts as a photoactive component (e.g.a photoabsorbent component or a photoemissive component) within thesemiconductor device. The semiconducting material may alternatively actas a p-type semiconductor component, an n-type semiconductor component,or an intrinsic semiconductor component in the semiconductor device. Forinstance, the semiconducting material may form a layer of a p-type,n-type or intrinsic semiconductor in a transistor, e.g. a field effecttransistor. For instance, the semiconducting material may form a layerof a p-type or n-type semiconductor in an optoelectronic device, e.g. asolar cell or an LED.

Typically, the semiconductor device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   a layer of said semiconducting material.

For instance, the semiconductor device is often an optoelectronicdevice, which optoelectronic device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   said layer of a semiconducting material which comprises (or        consists essentially of) a layer of said compound (e.g. a double        perovskite compound as defined herein).

The n-type region comprises at least one n-type layer. The n-type regiontypically comprises one or two n-type layers. Each layer may be porousor compact. A compact layer is typically a layer without open porosity(e.g. absent of any meso- or macroporosity). The p-type region comprisesat least one p-type layer. The p-type region typically comprises one ortwo p-type layers. Each layer may be porous or compact. A compact layeris typically a layer without open porosity.

In some cases, the semiconductor device comprises a layer of saidsemiconducting material without open porosity. The layer of saidsemiconducting material without open porosity is typically a layer of aperovskite compound according to the invention without open porosity.Thus, the layer of said semiconducting material may comprise greaterthan or equal to 95 volume % of the semiconducting material (and thusless than 5 volume % of absence pore volume). As described above, alayer without open porosity is a layer which typically does not comprisemacropores or mesopores.

The layer of the semiconducting material typically forms a planarheterojunction with the n-type region or the p-type region. The layer ofthe semiconducting material typically forms a first planarheterojunction with the n-type region and a second planar heterojunctionwith the p-type region. This forms a planar heterojunction device. Theterm “planar heterojunction” as used herein refers to a junction betweentwo regions where one region does not infiltrate the other. This doesnot require that the junction is completely smooth, just that one regiondoes not substantially infiltrate pores in the other region.

When the layer of the semiconducting material forms a planarheterojunction with both the p-type and the n-type region, thistypically forms a thin film device. The thickness of the layer of thesemiconducting material may be greater than or equal to 50 nm.Preferably, the thickness of the layer of the semiconducting material isgreater than or equal to 100 nm, for instance from 100 nm to 700 nm.

In some embodiments, it is desirable to have a porous scaffold materialpresent. The layer of a porous scaffold is usually in contact with acompact layer of a semiconductor material, for instance an n-typecompact layer or a p-type compact layer. The layer of a porous scaffoldis usually also in contact with the semiconducting material. Thescaffold material is typically mesoporous or macroporous. The scaffoldmaterial may aid charge transport from the semiconducting material to anadjacent region. The scaffold material may also, or alternatively, aidformation of the layer of the semiconducting material during deviceconstruction. The porous scaffold material is typically infiltrated bythe semiconducting material.

Thus, in some embodiments, the semiconductor device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   (i) a porous scaffold material; and    -   (ii) said semiconducting material in contact with the scaffold        material.

The semiconducting material in contact with the porous scaffold materialmay form a sensitizing layer of the semiconducting material. Thus, thesemiconducting device may be a halide double perovskite-sensitizeddevice.

Typically, the semiconducting material in the first layer is disposed inpores of the scaffold material. The scaffold material is typicallymesoporous. The scaffold material may be macroporous.

Typically, the porous scaffold material comprises a dielectric materialor a charge-transporting material. The scaffold material may be adielectric scaffold material. The scaffold material may be acharge-transporting scaffold material. The porous scaffold material maybe an electron-transporting material or a hole-transporting scaffoldmaterial. n-type semiconducting materials are examples ofelectron-transporting materials. p-type semiconductors are examples ofhole-transporting scaffold materials. Preferably, the porous scaffoldmaterial is a dielectric scaffold material or an electron-transportingscaffold material (e.g. an n-type scaffold material).

The porous scaffold material may be a charge-transporting scaffoldmaterial (e.g. an electron-transporting material such as titania, oralternatively a hole transporting material) or a dielectric material,such as alumina. The term “dielectric material”, as used herein, refersto material which is an electrical insulator or a very poor conductor ofelectric current. The term dielectric therefore excludes semiconductingmaterials such as titania. The term dielectric, as used herein,typically refers to materials having a band gap of equal to or greaterthan 4.0 eV. (The band gap of titania is about 3.2 eV.) The skilledperson of course is readily able to measure the band gap of asemiconductor (including that of a double perovskite compound) by usingwell-known procedures which do not require undue experimentation. Forinstance, the band gap of a semiconductor can be estimated byconstructing a photovoltaic diode or solar cell from the semiconductorand determining the photovoltaic action spectrum as described above.Alternatively the band gap can be estimated by measuring the lightabsorption spectra either via transmission spectrophotometry or by photothermal deflection spectroscopy. The band gap can be determined bymaking a Tauc plot, as described in Tauc, J., Grigorovici, R. & Vancu,a. Optical Properties and Electronic Structure of Amorphous Germanium.Phys. Status Solidi 15, 627-637 (1966) where the square of the productof absorption coefficient times photon energy is plotted on the Y-axisagainst photon energy on the x-axis with the straight line intercept ofthe absorption edge with the x-axis giving the optical band gap of thesemiconductor.

The porous scaffold material typically comprises an n-type semiconductoror a dielectric material. For instance, the device may comprise a layerof said porous scaffold material, where the porous scaffold materialcomprises an n-type semiconductor.

The porous scaffold is typically in the form of a layer. For instance,the porous scaffold may be a layer of porous scaffold material,typically having a thickness of from 5 nm to 500 nm, for instance from10 nm to 200 nm.

In some embodiments, the semiconductor device comprises:

-   -   an n-type region comprising at least one n-type layer;    -   a p-type region comprising at least one p-type layer; and,        disposed between the n-type region and the p-type region:    -   (i) a first layer which comprises a porous scaffold material and        said semiconducting material; and    -   (ii) a capping layer disposed on said first layer, which capping        layer is a layer of said semiconducting material without open        porosity,    -   wherein the semiconducting material in the capping layer is in        contact with the semiconducting material in the first layer.

The first layer comprises said porous scaffold material and saidsemiconducting material disposed on the surface of the scaffoldmaterial. The term “scaffold material” as used herein refers to amaterial whose function(s) include acting as a physical support foranother material. In the present case, the scaffold material acts as asupport for the semiconducting material present in the first layer. Thesemiconducting material is disposed, or supported on, the surface of thescaffold material. The porous scaffold material typically has an openporous structure. Accordingly, the “surface” of the porous scaffoldmaterial here typically refers to the surfaces of pores within thescaffold material. Thus, the semiconducting material in the first layeris typically disposed on the surfaces of pores within the scaffoldmaterial.

In some embodiments, the scaffold material is porous and thesemiconducting material in the first layer is disposed in pores of thescaffold material. The effective porosity of said scaffold material isusually at least 50%. For instance, the effective porosity may be about70%. In one embodiment, the effective porosity is at least 60%, forinstance at least 70%.

Typically, the semiconducting material (or photoactive material) in thefirst layer contacts one of the p-type and n-type regions, and thesemiconducting material in the capping layer contacts the other of thep-type and n-type regions. The semiconducting material in the cappinglayer typically forms a planar heterojunction with the p-type region orthe n-type region.

In one embodiment, the semiconducting material in the capping layercontacts the p-type region, and the semiconducting material in the firstlayer contacts the n-type region. In another embodiment, thesemiconducting material in the capping layer contacts the n-type region,and the semiconducting material in the first layer contacts the p-typeregion (for instance in an inverted device).

In one embodiment, the semiconducting material in the capping layercontacts the p-type region, and the semiconducting material in the firstlayer contacts the n-type region. Usually, in this embodiment, thescaffold material is either an electron-transporting scaffold materialor a dielectric scaffold material. Typically, the semiconductingmaterial in the capping layer forms a planar heterojunction with thep-type region.

In another embodiment, however, the semiconducting material in thecapping layer contacts the n-type region, and the semiconductingmaterial in the first layer contacts the p-type region. Typically, inthis embodiment, the scaffold material is a hole-transporting scaffoldmaterial or a dielectric scaffold material. Typically, thesemiconducting material in the capping layer forms a planarheterojunction with the n-type region.

The thickness of the capping layer is usually greater than the thicknessof the first layer. The majority of any photoactivity (e.g. lightabsorption or light emission) therefore usually occurs in a cappinglayer.

The thickness of the capping layer is typically from 10 nm to 100 μm.More typically, the thickness of the capping layer is from 10 nm to 10μm. Preferably, the thickness of the capping layer is from 50 nm to 1000nm, or for instance from 100 nm to 700 nm. The thickness of the cappinglayer may be greater than or equal to 100 nm.

The thickness of the first layer, on the other hand, is often from 5 nmto 1000 nm. More typically, it is from 5 nm to 500 nm, or for instancefrom 30 nm to 200 nm.

The n-type region is typically an n-type layer. The n-type region mayalternatively comprise an n-type layer and an n-type exciton blockinglayer. Such an n-type exciton blocking layer is typically disposedbetween the n-type layer and the layer(s) comprising the semiconductingmaterial. The n-type region may have a thickness of from 50 nm to 1000nm. For instance, the n-type region may have a thickness of from 50 nmto 500 nm, or from 100 nm to 500 nm.

Preferably, the n-type region comprises a compact layer of an n-typesemiconductor. The n-type region may further comprise a porous layer ofan n-type semiconductor which may be the porous scaffold material asdescribed above (wherein the porous scaffold material is anelectron-transporting material).

The n-type region in the optoelectronic device of the inventioncomprises one or more n-type layers. Often, the n-type region is ann-type layer, i.e. a single n-type layer. In other embodiments, however,the n-type region may comprise an n-type layer and an n-type excitonblocking layer. In cases where an n-type exciton blocking layer isemployed, the n-type exciton blocking layer is usually disposed betweenthe n-type layer and the layer(s) comprising the semiconductingmaterial.

An exciton blocking layer is a material which is of wider band gap thanthe semiconducting material, but has either its conduction band orvalance band closely matched with those of the semiconducting material.If the conduction band (or lowest unoccupied molecular orbital energylevels) of the exciton blocking layer are closely aligned with theconduction band of the semiconducting material, then electrons can passfrom the semiconducting material into and through the exciton blockinglayer, or through the exciton blocking layer and into the semiconductingmaterial, and we term this an n-type exciton blocking layer. An exampleof such is bathocuproine, as described in P. Peumans, A. Yakimov, and S.R. Forrest, “Small molecular weight organic thin-film photodetectors andsolar cells” J. Appl. Phys. 93, 3693 (2001) and Masaya Hirade, andChihaya Adachi, “Small molecular organic photovoltaic cells with excitonblocking layer at anode interface for improved device performance” Appl.Phys. Lett. 99, 153302 (2011)}.

An n-type layer is a layer of an electron-transporting (i.e. an n-type)material. The n-type material may, for instance, be a single n-typecompound or elemental material, which may be undoped or doped with oneor more dopant elements.

The n-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic n-type material.

A suitable inorganic n-type material may be selected from a metal oxide,a metal sulphide, a metal selenide, a metal telluride, a perovskite,amorphous Si, an n-type group IV semiconductor, an n-type group III-Vsemiconductor, an n-type group II-VI semiconductor, an n-type groupI-VII semiconductor, an n-type group IV-VI semiconductor, an n-typegroup V-VI semiconductor, and an n-type group II-V semiconductor, any ofwhich may be doped or undoped.

The n-type material may be selected from a metal oxide, a metalsulphide, a metal selenide, a metal telluride, amorphous Si, an n-typegroup IV semiconductor, an n-type group III-V semiconductor, an n-typegroup II-VI semiconductor, an n-type group I-VII semiconductor, ann-type group IV-VI semiconductor, an n-type group V-VI semiconductor,and an n-type group II-V semiconductor, any of which may be doped orundoped.

More typically, the n-type material is selected from a metal oxide, ametal sulphide, a metal selenide, and a metal telluride.

Thus, the n-type layer may comprise an inorganic material selected fromoxide of titanium, tin, zinc, niobium, tantalum, tungsten, indium,gallium, neodymium, palladium, or cadmium, or an oxide of a mixture oftwo or more of said metals. For instance, the n-type layer may compriseTiO₂, SnO₂, ZnO, Nb₂O₅, Ta₂O₅, WO₃, W₂O₅, In₂O₃, Ga₂O₃, Nd₂O₃, PbO, orCdO.

Other suitable n-type materials that may be employed include sulphidesof cadmium, tin, copper, or zinc, including sulphides of a mixture oftwo or more of said metals. For instance, the sulphide may be FeS₂, CdS,ZnS, SnS, BiS, SbS, or Cu₂ZnSnS₄.

The n-type layer may for instance comprise a selenide of cadmium, zinc,indium, or gallium or a selenide of a mixture of two or more of saidmetals; or a telluride of cadmium, zinc, cadmium or tin, or a tellurideof a mixture of two or more of said metals. For instance, the selenidemay be Cu(In,Ga)Se₂. Typically, the telluride is a telluride of cadmium,zinc, cadmium or tin. For instance, the telluride may be CdTe.

The n-type layer may for instance comprise an inorganic materialselected from oxide of titanium, tin, zinc, niobium, tantalum, tungsten,indium, gallium, neodymium, palladium, cadmium, or an oxide of a mixtureof two or more of said metals; a sulphide of cadmium, tin, copper, zincor a sulphide of a mixture of two or more of said metals; a selenide ofcadmium, zinc, indium, gallium or a selenide of a mixture of two or moreof said metals; or a telluride of cadmium, zinc, cadmium or tin, or atelluride of a mixture of two or more of said metals.

Examples of other semiconductors that may be suitable n-type materials,for instance if they are n-doped, include group IV elemental or compoundsemiconductors; amorphous Si; group III-V semiconductors (e.g. galliumarsenide); group II-VI semiconductors (e.g. cadmium selenide); groupI-VII semiconductors (e.g. cuprous chloride); group IV-VI semiconductors(e.g. lead selenide); group V-VI semiconductors (e.g. bismuthtelluride); and group II-V semiconductors (e.g. cadmium arsenide).

Typically, the n-type layer comprises TiO₂.

When the n-type layer is an inorganic material, for instance TiO₂ or anyof the other materials listed above, it may be a compact layer of saidinorganic material. Preferably the n-type layer is a compact layer ofTiO₂.

Other n-type materials may also be employed, including organic andpolymeric electron-transporting materials, and electrolytes. Suitableexamples include, but are not limited to a fullerene or a fullerenederivative (for instance C₆₀ or Phenyl-C61-butyric acid methyl ester(PCBM)), an organic electron transporting material comprising peryleneor a derivative thereof, or poly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI2OD-T2)).

The p-type region is typically a p-type layer. The p-type region mayalternatively comprise an p-type layer and a p-type exciton blockinglayer. Such a p-type exciton blocking layer is typically disposedbetween the p-type layer and the layer(s) comprising the semiconductingmaterial. The p-type region may have a thickness of from 50 nm to 1000nm. For instance, the p-type region may have a thickness of from 50 nmto 500 nm, or from 100 nm to 500 nm.

The p-type region in the semiconductor device of the invention comprisesone or more p-type layers. Often, the p-type region is a p-type layer,i.e. a single p-type layer. In other embodiments, however, the p-typeregion may comprise a p-type layer and a p-type exciton blocking layer.In cases where a p-type exciton blocking layer is employed, the p-typeexciton blocking layer is usually disposed between the p-type layer andthe layer(s) comprising the semiconducting material. If the valence band(or highest occupied molecular orbital energy levels) of the excitonblocking layer is closely aligned with the valence band of thesemiconducting material, then holes can pass from the semiconductingmaterial into and through the exciton blocking layer, or through theexciton blocking layer and into the semiconducting material, and we termthis a p-type exciton blocking layer. An example of such istris[4-(5-phenylthiophen-2-yl)phenyl]amine, as described in MasayaHirade, and Chihaya Adachi, “Small molecular organic photovoltaic cellswith exciton blocking layer at anode interface for improved deviceperformance” Appl. Phys. Lett. 99, 153302 (2011).

A p-type layer is a layer of a hole-transporting (i.e. a p-type)material. The p-type material may be a single p-type compound orelemental material, or a mixture of two or more p-type compounds orelemental materials, which may be undoped or doped with one or moredopant elements.

The p-type layer employed in the optoelectronic device of the inventionmay comprise an inorganic or an organic p-type material. Typically, thep-type region comprises a layer of an organic p-type material.

Suitable p-type materials may be selected from polymeric or molecularhole transporters. The p-type layer employed in the optoelectronicdevice of the invention may for instance comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),PVK (poly(N-vinylcarbazole)), HTM-TFSI (1-hexyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide), Li-TFSI (lithiumbis(trifluoromethanesulfonyl)imide) or tBP (tert-butylpyridine). Thep-type region may comprise carbon nanotubes. Usually, the p-typematerial is selected from spiro-OMeTAD, P3HT, PCPDTBT and PVK.Preferably, the p-type layer employed in the optoelectronic device ofthe invention comprises spiro-OMeTAD.

The p-type layer may for example comprise spiro-OMeTAD(2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene)),P3HT (poly(3-hexylthiophene)), PCPDTBT(Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]]),or PVK (poly(N-vinylcarbazole)).

Suitable p-type materials also include molecular hole transporters,polymeric hole transporters and copolymer hole transporters. The p-typematerial may for instance be a molecular hole transporting material, apolymer or copolymer comprising one or more of the following moieties:thiophenyl, phenelenyl, dithiazolyl, benzothiazolyl,diketopyrrolopyrrolyl, ethoxydithiophenyl, amino, triphenyl amino,carbozolyl, ethylene dioxythiophenyl, dioxythiophenyl, or fluorenyl.Thus, the p-type layer employed in the optoelectronic device of theinvention may for instance comprise any of the aforementioned molecularhole transporting materials, polymers or copolymers.

Suitable p-type materials also include m-MTDATA(4,4′,4″-tris(methylphenylphenylamino)triphenylamine), MeOTPD(N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine), BP2T(5,5′-di(biphenyl-4-yl)-2,2′-bithiophene), Di-NPB(N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine),α-NPB (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine), TNATA(4,4′,4″-tris-(N-(naphthylen-2-yl)-N-phenylamine)triphenylamine), BPAPF(9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene), spiro-NPB(N2,N7-Di-1-naphthalenyl-N2,N7-diphenyl-9,9′-spirobi[9H-fluorene]-2,7-diamine),4P-TPD (4,4-bis-(N,N-diphenylamino)-tetraphenyl), PEDOT:PSS andspiro-OMeTAD.

The p-type layer may be doped, for instance with tertbutyl pyridine andLiTFSI. The p-type layer may be doped to increase the hole-density. Thep-type layer may for instance be doped with NOBF₄ (Nitrosoniumtetrafluoroborate), to increase the hole-density.

In other embodiments, the p-type layer may comprise an inorganic holetransporter. For instance, the p-type layer may comprise an inorganichole transporter comprising an oxide of nickel, vanadium, copper ormolybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO or CIS; a perovskite; amorphousSi; a p-type group IV semiconductor, a p-type group III-V semiconductor,a p-type group II-VI semiconductor, a p-type group I-VII semiconductor,a p-type group IV-VI semiconductor, a p-type group V-VI semiconductor,and a p-type group II-V semiconductor, which inorganic material may bedoped or undoped. The p-type layer may be a compact layer of saidinorganic hole transporter.

The p-type layer may for instance comprise an inorganic hole transportercomprising an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.The p-type layer may for instance comprise an inorganic hole transporterselected from CuI, CuBr, CuSCN, Cu₂O, CuO and CIS. The p-type layer maybe a compact layer of said inorganic hole transporter.

Typically, the p-type layer comprises a polymeric or molecular holetransporter, and the n-type layer comprises an inorganic n-typematerial. The p-type polymeric or molecular hole transporter may be anysuitable polymeric or molecular hole transporter, for instance any ofthose listed above. Likewise, the inorganic n-type material may be anysuitable n-type inorganic, for instance any of those listed above. Inone embodiment, for instance, the p-type layer comprises spiro-OMeTADand the n-type layer comprises TiO₂. Typically, in that embodiment, then-type layer which comprises TiO₂ is a compact layer of TiO₂.

In other embodiments, both the n-type layer and the p-type layercomprise inorganic materials. Thus, the n-type layer may comprise aninorganic n-type material and the p-type layer may comprise an inorganicp-type material. The inorganic p-type material may be any suitablep-type inorganic, for instance any of those listed above. Likewise, theinorganic n-type material may be any suitable n-type inorganic, forinstance any of those listed above.

In other embodiments, the p-type layer comprises an inorganic p-typematerial (i.e. an inorganic hole transporter) and the n-type layercomprises a polymeric or molecular hole transporter. The inorganicp-type material may be any suitable p-type inorganic, for instance anyof those listed above. Likewise, the n-type polymeric or molecular holetransporter may be any suitable n-type polymeric or molecular holetransporter, for instance any of those listed above.

For instance, the p-type layer may comprise an inorganic holetransporter and the n-type layer may comprise an electron transportingmaterial, wherein the electron transporting material comprises afullerene or a fullerene derivative, an electrolyte, or an organicelectron transporting material, preferably wherein the organic electrontransporting material comprises perylene or a derivative thereof, orpoly{[N,N0-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,50-(2,20-bithiophene)}(P(NDI20D-T2)). The inorganic hole transporter may for instance comprisean oxide of nickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN,Cu₂O, CuO or CIS; a perovskite; amorphous Si; a p-type group IVsemiconductor, a p-type group III-V semiconductor, a p-type group II-VIsemiconductor, a p-type group I-VII semiconductor, a p-type group IV-VIsemiconductor, a p-type group V-VI semiconductor, and a p-type groupII-V semiconductor, which inorganic material may be doped or undoped.More typically, the inorganic hole transporter comprises an oxide ofnickel, vanadium, copper or molybdenum; CuI, CuBr, CuSCN, Cu₂O, CuO orCIS; a p-type group IV semiconductor, a p-type group III-Vsemiconductor, a p-type group II-VI semiconductor, a p-type group I-VIIsemiconductor, a p-type group IV-VI semiconductor, a p-type group V-VIsemiconductor, and a p-type group II-V semiconductor, which inorganicmaterial may be doped or undoped. Thus, the inorganic hole transportermay comprise an oxide of nickel, vanadium, copper or molybdenum; CuI,CuBr, CuSCN, Cu₂O, CuO or CIS.

The semiconductor device typically further comprises one or more firstelectrodes and one or more second electrodes. The one or more firstelectrodes are typically in contact with the n-type region, if such aregion is present. The one or more second electrodes are typically incontact with the p-type region, if such a region is present. Typically:the one or more first electrodes are in contact with the n-type regionand the one or more second electrodes are in contact with the p-typeregion; or the one or more first electrodes are in contact with thep-type region and the one or more second electrodes are in contact withthe n-type region.

The first and second electrode may comprise any suitable electricallyconductive material. The first electrode typically comprises atransparent conducting oxide. The second electrode typically comprisesone or more metals. Typically, the first electrode typically comprises atransparent conducting oxide and the second electrode typicallycomprises one or more metals.

The transparent conducting oxide may be as defined above and is oftenFTO, ITO, or AZO, and typically ITO. The metal may be any metal.Generally the second electrode comprises a metal selected from silver,gold, copper, aluminium, platinum, palladium, or tungsten. Theelectrodes may form a single layer or may be patterned.

A semiconductor device according to the invention, for instance asensitized solar cell, may comprise the following layers in thefollowing order:

-   -   I. one or more first electrodes as defined herein;    -   II. optionally a compact n-type layer as defined herein;    -   III. a porous layer of an n-type material as defined herein;    -   IV. a layer of said semiconducting material (e.g. as a        sensitizer);    -   V. a p-type region as defined herein;    -   VI. optionally a further compact p-type layer as defined herein;        and    -   VII. one or more second electrodes as defined herein.

A semiconductor device according to the invention which is aphotovoltaic device may comprise the following layers in the followingorder:

-   -   I. one or more first electrodes as defined herein;    -   II. an n-type region comprising at least one n-type layer as        defined herein;    -   III. a layer of the semiconducting material comprising the        compound as defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more second electrodes as defined herein.

A photovoltaic device according to the invention may comprise thefollowing layers in the following order:

-   -   I. one or more first electrodes which comprise a transparent        conducting oxide, preferably FTO;    -   II. an n-type region comprising at least one n-type layer as        defined herein;    -   III. a layer of the semiconducting material as defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more second electrodes which comprise a metal,        preferably silver or gold.

A photovoltaic device (for instance an inverted device) according to theinvention may comprise the following layers in the following order:

-   -   I. one or more second electrodes as defined herein;    -   II. an n-type region comprising at least one n-type layer as        defined herein;    -   III. a layer of the semiconducting material as defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more first electrodes as defined herein.

A photovoltaic device according to the invention, for instance asensitized solar cell, may comprise the following layers in thefollowing order

-   -   I. one or more second electrodes which comprises a metal;    -   II. an n-type region comprising at least one mesoporous n-type        layer as defined herein;    -   III. a sensitising layer of the semiconducting material as        defined herein;    -   IV. a p-type region comprising at least one p-type layer as        defined herein; and    -   V. one or more first electrodes which comprise a transparent        conducting oxide.

The one or more first electrodes may have a thickness of from 100 nm to700 nm, for instance of from 100 nm to 400 nm. The one or more secondelectrodes may have a thickness of from 10 nm to 500 nm, for instancefrom 50 nm to 200 nm or from 10 nm to 50 nm. The n-type region may havea thickness of from 50 nm to 500 nm. The p-type region may have athickness of from 50 nm to 500 nm.

Process for Producing a Semiconductor Device

The invention also provides a process for producing a semiconductordevice comprising a semiconducting material, wherein the semiconductingmaterial comprises a compound comprising: (i) one or more firstmonocations [A]; (ii) one or more second monocations [B^(I)]; (iii) oneor more trications [B^(III)]; and (iv) one or more halide anions [X],which process comprises: (a) disposing a second region on a firstregion, which second region comprises a layer of said semiconductingmaterial. The semiconducting material may be as further defined herein.For instance, the semiconducting material may comprise a doubleperovskite as defined herein, for instance a double perovskite offormula (I) or formula (Ia) above.

The second region may be disposed by vapour deposition. Thus, (i)disposing a second region on a first region may comprise:

-   -   (Ai) exposing the first region to vapour, which vapour comprises        said semiconducting material or one or more reactants for        producing said semiconducting material; and    -   (Aii) allowing deposition of the vapour onto the first region to        produce a layer of said semiconducting material thereon.

The vapour deposition process generally further comprises producing thevapour in the first place by evaporating said semiconducting material orevaporating said one or more reactants for producing said semiconductingmaterial. In this step the semiconducting material or the one or morereactants for producing the semiconducting material are typicallytransferred to an evaporation chamber which is subsequently evacuated.The semiconducting material or the one or more reactants for producingthe semiconducting material are typically then heated to produce aresulting vapour.

The resulting vapour is then exposed to and thereby deposited on thefirst region, to produce a solid layer of said semiconducting materialthereon. If reactants are used, these may react together in situ toproduce the semiconducting material on the first region.

Typically, the vapour deposition is allowed to continue until the layerof semiconducting material has a desired thickness, for instance athickness of from 10 nm to 100 m, or more typically from 10 nm to 10 μm.Preferably, the vapour deposition is allowed to continue until the layerof semiconducting material has a thickness of from 50 nm to 1000 nm, orfor instance from 100 nm to 700 nm. For instance, deposition may becontinued until approximately 100 nm to 300 nm of the powder isdeposited onto the first region.

The vapour deposition may continue until the layer of the semiconductingmaterial has a thickness of at least 100 nm. Typically, for instance, itcontinues until the solid layer of the semiconducting material has athickness of from 100 nm to 100 μm, or for instance from 100 nm to 700nm.

In one embodiment, the step disposing the second region on the firstregion comprises:

-   -   (i) exposing the first region to vapour, which vapour comprises        one or more first precursor compounds, one or more second        precursor compounds and one or more third precursor compounds;        and    -   (ii) allowing deposition of the vapour onto the first region, to        produce a layer of said semiconducting material thereon;    -   wherein (i) further comprises producing said vapour comprising        one or more first precursor compounds, one or more second        precursor compounds and one or more third precursor compounds by        evaporating the one or more first precursor compounds and one or        more second precursor compounds from a first source and        evaporating one or more third precursor compounds from a second        source. The one or more first precursor compounds, one or more        second precursor compounds and one or more third precursor        compounds may be as defined herein. The two sources are        typically placed at the same distance from the first region,        often from 10 to 40 cm.

The second region may instead be disposed by solution processing. Thus,(i) disposing a second region on a first region may comprise:

-   -   (Bi) disposing one or more precursor compositions on the first        region, which one or more precursor compositions comprise: said        semiconducting material and one or more solvents; or one or more        reactants for producing said semiconducting material and one or        more solvents; and    -   (Bii) removing the one or more solvents to produce on the first        region a layer of said semiconducting material.

Said semiconducting material may be as defined herein. Thus, thesemiconducting material is often a photoactive material, for instance aphotoabsorbent material or a photoemissive material. The one or moreprecursor compositions may comprise said semiconducting material asdefined herein and one or more solvents.

The one or more reactants for producing said semiconducting material maybe as defined below. The one or more solvents may be any suitablesolvents. Typically the one or more solvents are selected from polarsolvents. Examples of polar solvents include water, alcohol solvents(such as methanol, ethanol, n-propanol, isopropanol and n-butanol),ether solvents (such as dimethylether, diethylether andtetrahydrofuran), ester solvents (such as ethyl acetate), carboxylicacid solvents (such as formic acid and ethanoic acid), ketone solvents(such as acetone), amide solvents (such as dimethylformamide anddiethylformamide), amine solvents (such as triethylamine), nitrilesolvents (such as acetonitrile), sulfoxide solvents (dimethylsulfoxide)and halogenated solvents (such as dichloromethane, chloroform, andchlorobenzene). The one or more solvents may be selected from polaraprotic solvents. Examples of protic apolar solvents includedimethylformamide (DMF), acetonitrile and dimethylsulfoxide (DMSO).Preferably the one or more solvents are a single polar solvent, forinstance DMF, DMSO, ethanol or isopropanol.

Usually, the steps of (Bi) disposing a precursor solution on the firstregion, and (Bii) removing the solvent, comprise spin-coating orslot-dye-coating the precursor solution or solutions onto the firstregion, to produce on the first region a layer of the semiconductingmaterial. Said coating may be carried out in an inert atmosphere, forinstance under nitrogen, or it may be carried out in air. Thespin-coating is usually performed at a speed of from 1000 to 3000 rpm.The spin coating is typically carried out for 30 seconds to 2 minutes.

Typically, (Bi) comprises disposing a solution of a first precursorcompound (for instance methylammonium iodide), a second precursorcompound (for instance silver iodide) and a third precursor compound(for instance bismuth triiodide) in a polar solvent by spin-coating.(Bi) may, for instance, comprise disposing a solution of the perovskitecompound in a polar solvent by spin-coating. The solution of theperovskite compound in a polar solvent may be produced by dissolvingpowder of the perovskite compound in the polar solvent.

The precursor solution or solutions may be disposed by spin-coating ontothe first region to produce on the first region said layer of thesemiconducting material.

The steps of disposing the precursor solution or solutions on the firstregion and removing the solvent or solvents are carried out until thelayer of the semiconducting material has a desired thickness, forinstance a thickness of from 10 nm to 100 μm, more typically from 10 nmto 10 μm. The thickness of the layer of the semiconducting material maybe as described above. For instance, the steps of disposing theprecursor solution or solutions on the first region and removing thesolvent or solvents may be carried out until the layer of thesemiconducting material has a thickness of from 50 nm to 1000 nm, or forinstance from 100 nm to 700 nm. The layer of the semiconducting materialmay have a thickness of greater than or equal to 100 nm.

Removing the one or more solvents typically comprises heating the one ormore solvents or allowing the one or more solvents to evaporate. Thesubstrate, solvent or first region may be heated at a temperature offrom 40° C. to 100° C. for a time of from 5 minutes to 2 hours to removethe one or more solvents.

The one or more reactants for producing the semiconducting materialtypically comprise one or more first precursor compounds, one or moresecond precursor compounds and one or more third precursor compounds,which one or more first precursor compounds are selected from compoundsof formula [A][X];

-   -   which one or more second precursor compounds are selected from        compounds of formula [B^(I])[X]; and    -   which one or more second precursor compounds are selected from        compounds of formula [B^(III)][X]₃;    -   wherein [A] is the one or more first monocations; [B^(I)] is the        one or more second monocations; [B^(III)] is the one or more        trications; and each [X] is the one or more halide anions. [A],        [B^(I)], [B^(III)] and [X] each may be as defined herein.

Typically, the one or more first precursor compounds are selected from(CH₃NH₃)I, (CH₃NH₃)Br, (CH₃NH₃)Cl, (CH₃NH₃)F, (H₂N—C(H)═NH₂)I,(H₂N—C(H)═NH₂)Br, (H₂N—C(N)═NH₂)Cl, (H₂N—C(N)═NH₂)F, (H₂N—C(NH₂)═NH₂)I,(H₂N—C(NH₂)═NH₂)Br, (H₂N—C(NH₂)═NH₂)Cl, (H₂N—C(NH₂)═NH₂)F, CsI, CsBr,CsCl and CsF. Preferably, the one or more first precursor compounds areselected from (CH₃NH₃)I, (CH₃NH₃)Br, (H₂N—C(H)═NH₂)I and(H₂N—C(H)═NH₂)Br, more preferably selected from (CH₃NH₃)Iand(H₂N—C(H)═NH₂).

Typically, the one or more second precursor compounds are selected fromcompounds of formula AgI, AgBr, AgCl, AgF, AuI, AuBr, AuCl, AuF, CuI,CuBr, CuCl and CuF. Preferably, the one or more second precursorcompounds are selected from compounds of formula AgI, AgBr, AuI, AuBr,CuI and CuBr, more preferably selected from AgI and AuI.

Typically, the one or more second precursor compounds are selected fromcompounds of formula BiI₃, BiBr₃, BiCl₃, BiF₃, Sb₃, SbBr₃, SbCl₃ andSbF₃. Preferably, the one or more second precursor compounds areselected from compounds of formula BiI₃, BiBr₃, Sb₃ and SbBr₃, morepreferably selected from BiI₃ and SbI₃.

For instance the first precursor compound may be (CH₃NH₃)I, (CH₃NH₃)Br,(H₂N—C(H)═NH₂)I or (H₂N—C(H)═NH₂)Br, the second precursor compound maybe AgI, AgBr, AuI, AuBr, CuI or CuBr, and the third precursor compoundmay be BiI₃, BiBr₃, Sb₃ or SbBr₃.

After the second region is deposited, the process may further comprise astep of annealing the second region. For instance, the second region maybe heated to a temperature of from 50° C. to 200° C., or from 70° C. to150° C. The second region may be heated to a temperature of from 90° C.to 110° C. The second region may be heated for a time from 30 seconds to60 minutes, for instance from 2 minutes to 25 minutes.

Typically, the process further comprises (b) disposing a third region onthe second region, wherein: said first region is an n-type regioncomprising at least one n-type layer and said third region is a p-typeregion comprising at least one p-type layer; or said first region is ap-type region comprising at least one p-type layer and said third regionis an n-type region comprising at least one n-type layer.

The third region is typically a p-type region comprising at least onep-type layer, preferably wherein the at least one p-type layer comprisesan organic p-type semiconductor. The p-type region may be as describedabove.

The third region is typically disposed on the second region until it hasa thickness of from 50 nm to 1000 nm, for instance 100 nm to 500 nm.Disposing the third region on the second region typically comprisesdisposing a composition comprising a p-type material and a solvent onthe second region (for instance by spin-coating) and removing thesolvent. The p-type material may be any p-type material describedherein. Preferably, said third region is a p-type region comprising atleast one p-type layer, preferably wherein the at least one p-type layercomprises an organic p-type material, for instance spiro-OMeTAD.

The process typically further comprises: (c) disposing one or moresecond electrodes on the third region. The one or more second electrodesmay be as defined above for an semiconductor device according to theinvention. For instance, the second electrodes may comprise a metal suchas silver. The one or more second electrodes are typically disposed byvacuum vapour deposition, for instance by evaporation at a low pressure(e.g less than or equal to 10⁻⁵ mbar) optionally through a shadow mask.

An exemplary process according to the invention may be as follows: (i)separately dissolve each of (1) MAX or FAX powder, (2) BiX₃ or SbX₃powder and (3) AgX or AuI powder in a solvent, for instance DMF, at aconcentration of from 10 wt % to 40 wt % where X is I, Br, Cl or F; (ii)agitate each solution until it fully dissolves (optionally includingheating to from 50° C. to 120° C.); (iii) combine the three solutions toproduce a precursor solution; (iv) provide a substrate (e.g. glass/FTOwith titania or a microscope slide); (v) spin coat the precursorsolution on the substrate; (vi) anneal at from 80° C. to 120° C. forfrom 1 min to 20 min; (vii) dispose a p-type layer, for instancespiro-OMeTAD, by spin-coating; and (vii) deposit metal, for instancesilver, electrodes to form the device.

The invention also provides a semiconductor device obtainable by aprocess for producing a semiconductor device according to the invention.

Compound

The invention provides a compound comprising: (i) one or more firstmonocations [A]; (ii) one or more second monocations [B^(I)] selectedfrom Cu⁺, Ag⁺ and Au⁺; (iii) one or more trications [B^(III)]; and (iv)one or more halide anions [X]. The compound is typically crystalline.The compound may be polycrystalline. The compound is usually solid.

The invention also provides a composition comprising greater than 0.1%of the compound according to the invention by weight relative to theweight of the total composition. The composition may comprise greaterthan 1.0% by weight or greater than 5.0% by weight. The invention alsoprovides a composition comprising greater than 10% by weight of thecompound according to the invention. The composition may comprisegreater than 50%, greater than 90% or greater than 95% by weight of thecompound of the invention. The composition may consist, or consistessentially of, the compound of the invention.

The compound is typically a double perovskite compound of formula (I) ora layered double perovskite compound of formula (II):

[A]₂[B^(I)][B^(III)][X]₆  (I);

[A]₄[B^(I)][B^(III)][X]₈  (II);

wherein: [A] is the one or more first monocations; [B^(I)] is the one ormore second monocations selected from Cu⁺, Ag⁺ and Au⁺; [B^(III)] is theone or more trications; and [X] is the one or more halide anions.

The compound may be as further defined herein.

The compound may be (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂CuBiI₆, (H₂N—C(H)═NH₂)₂AgSbI₆, (H₂N—C(H)═NH₂)₂AuSbI₆,(H₂N—C(H)═NH₂)₂CuSbI₆, (H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆,(H₂N—C(H)═NH₂)₂CuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(H₂N—C(H)═NH₂)₂CuSbBr₆, (H₂N—C(H)═NH₂)₂AgBiCl₆, (H₂N—C(H)═NH₂)₂AuBiCl₆,(H₂N—C(H)═NH₂)₂CuBiCl₆, (H₂N—C(H)═NH₂)₂AgSbCl₆, (H₂N—C(H)═NH₂)₂AuSbCl₆,(H₂N—C(H)═NH₂)₂CuSbCl₆, (H₂N—C(H)═NH₂)₂AgBiF₆, (H₂N—C(H)═NH₂)₂AuBiF₆,(H₂N—C(H)═NH₂)₂CuBiF₆, (H₂N—C(H)═NH₂)₂AgSbF₆, (H₂N—C(H)═NH₂)₂AuSbF₆,(H₂N—C(H)═NH₂)₂CuSbF₆, (CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆,(CH₃NH₃)₂CuBiI₆, (CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂CuSbI₆,(CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆, (CH₃NH₃)₂CuBiBr₆, (CH₃NH₃)₂AgSbBr₆,(CH₃NH₃)₂AuSbBr₆, (CH₃NH₃)₂CuSbBr₆, (CH₃NH₃)₂AgBiCl₆, (CH₃NH₃)₂AuBiCl₆,(CH₃NH₃)₂CuBiCl₆, (CH₃NH₃)₂AgSbCl₆, (CH₃NH₃)₂AuSbCl₆, (CH₃NH₃)₂CuSbCl₆,(CH₃NH₃)₂AgBiF₆, (CH₃NH₃)₂AuBiF₆, (CH₃NH₃)₂CuBiF₆, (CH₃NH₃)₂AgSbF₆,(CH₃NH₃)₂AuSbF₆, (CH₃NH₃)₂CuSbF₆, Cs₂AgBiI₆, Cs₂AuBiI₆, Cs₂CuBiI₆,Cs₂AgSbI₆, Cs₂AuSbI₆, Cs₂CuSbI₆, Cs₂AgBiBr₆, Cs₂AuBiBr₆, Cs₂CuBiBr₆,Cs₂AgSbBr₆, Cs₂AuSbBr₆, Cs₂CuSbBr₆, Cs₂AgBiCl₆, Cs₂AuBiCl₆, Cs₂CuBiCl₆,Cs₂AgSbCl₆, Cs₂AuSbCl₆, Cs₂CuSbCl₆, Cs₂AgBiF₆, Cs₂AuBiF₆, Cs₂CuBiF₆,Cs₂AgSbF₆, Cs₂AuSbF₆ or Cs₂CuSbF₆.

The compound is preferably (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbI₆,(H₂N—C(H)═NH₂)₂AuSbI₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆, (CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆,(CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂AgSbBr₆ or (CH₃NH₃)₂AuSbBr₆.

The compound is more preferably (H₂N—C(H)═NH₂)₂AgBiI₆,(H₂N—C(H)═NH₂)₂AuBiI₆, (CH₃NH₃)₂AgBiI₆ or (CH₃NH₃)₂AuBiI₆.Alternatively, the compound may be Cs₂AgBiCl₆.

The compounds of the invention may be synthesised by the followingmethod comprising: (i) dissolving one or more first precursor compoundsof formula [A][X]; one or more second precursor compounds of formula[B^(I)][X]; and one or more third precursor compounds of formula[B^(III)][X]₃; in a solvent; and (ii) removing the solvent (for instanceby ambient evaporation or heating) to produce the compound, typically adouble perovskite compound as defined herein; wherein [A] is the one ormore first monocations; [B^(I)] is the one or more second monocations;[B^(III)] is the one or more trications; and each [X] is the one or morehalide anions. [A], [B^(I)], [B^(III)] and [X] each may be as definedherein. The solvent may be as defined above and may, for instance,comprise dimethylformamide (DMF), acetonitrile or dimethylsulfoxide(DMSO). The precursor compounds and solvent may be heated in order topromote dissolution of the precursor compounds.

For example: a compound according to the invention may be synthesised by(i) separately dissolving each of (1) MAX or FAX powder, (2) BiX₃ orSbX₃ powder and (3) AgX or AuX (e.g. AuI) powder in a solvent, forinstance DMF, each at a concentration of from 10 wt % to 40 wt % where Xis I, Br, Cl or F; (ii) agitating each solution until it fully dissolves(optionally including heating to from 50° C. to 120° C.); (iii)combining the three solutions to produce a precursor solution; (iv)providing a substrate (e.g. glass/FTO with titania or a microscopeslide); (v) spin coat the precursor solution on the substrate; and (vi)anneal at from 80° C. to 120° C. for from 1 min to 20 min.

Typically, the three precursor compounds are combined in a molar ratioof B^(I)X:B^(III)X₃:AX of approximately 1:1:2. For instance, for eachequivalent of B^(I)X, there may be from 0.2 to 5.0 equivalents ofB^(III)X₃, preferably from 0.5 to 2.0 equivalents of B^(III)X₃, and from0.5 to 10.0 equivalents of AX, preferably from 1.0 to 3.0 equivalents ofAX.

The compounds described herein may alternatively be synthesised by asolid state process. The solid state process typically comprisescombining solid forms of one or more first precursor compounds offormula [A][X]; one or more second precursor compounds of formula[B^(I)][X]; and one or more third precursor compounds of formula[B^(III)][X]₃; and (ii) heating the combined solid forms of the first,second and third precursor compounds to produce the compound, typicallya double perovskite compound as defined herein. [A] is the one or morefirst monocations; [B^(I)] is the one or more second monocations;[B^(III)] is the one or more trications; and each [X] is the one or morehalide anions. [A], [B^(I)], [B^(III)] and [X] each may be as definedherein. Typically, the one or more first precursor compounds are asingle first precursor compound of formula AX; the one or more secondprecursor compounds are a single second precursor compound of formulaB^(I)X; and the one or more third precursor compounds are a single thirdprecursor compound of formula B^(III)X₃. For instance, the firstprecursor compound may be CsX; the second precursor compound may be AgXor AuX; and the third precursor compound may be SbX₃ or BiX₃. Typically,the three precursor compounds are combined in a molar ratio ofB^(I)X:B^(III)X₃:AX of approximately 1:1:2. For instance, for eachequivalent of B^(I)X, there may be from 0.2 to 5.0 equivalents ofB^(III)X₃, preferably from 0.5 to 2.0 equivalents of B^(III)X₃, and from0.5 to 10.0 equivalents of AX, preferably from 1.0 to 3.0 equivalents ofAX.

Typically, the solid forms of the precursor compounds are powders of theprecursor compounds. The solid forms of the precursor compounds aretypically combined by mixing together powders of those compounds. Theprecursor compounds are typically combined in a sealed container (forinstance a fused ampoule). The sealed container typically has a lowinterior pressure, for instance of less than or equal to 10⁻² Torr. Insome cases, the sealed container will be substantially free of oxygen,for instance having a partial pressure of less than or equal to 10⁻³Torr of oxygen.

Heating the combined solid forms of the first, second and thirdprecursor compounds is typically done by heating the combined solidforms to a temperature of greater than or equal to 100° C., for instancegreater than or equal to 300° C. For instance, the combined solid formsmay be heated to a temperature of from 300° C. to 800° C. Once heated,the combined solid forms may be held at the temperature to which theywere heated for from 0.5 to 10 hours, for instance from 3 to 6 hours.Preferably, the combined solid forms are heated to from 400° C. to 600°C. and held at that temperature from 2 to 6 hours. After heating, theresultant composition is typically then cooled to ambient temperature(e.g. 25° C.). Crystallisation of the double perovskite compound istypically then observed. Thin films of the perovskite compound may thenbe produced by dissolving the obtained double perovskite compound in asolvent (for instance DMF) to form a solution, and spin-coating thesolution on a substrate. Removal of the solvent typically leads toformation of a thin film.

The invention will be described further by the following Examples.

EXAMPLES Example 1—Computational Study Computational Setup

Structural optimisations were performed using DFT/LDA calculations,planewaves, and pseudopotentials, as implemented in the Quantum ESPRESSOdistribution. For Cu, Ag, Au, and C, N, H, we use ultrasoftpseudopotentials including nonlinear core correction. For Bi and Sb weuse norm-conserving pseudopotentials as in (Filip et al, quasiparticleband structures of stibnite, antimonselite, bismuthinite, andguanajuatite. Phys. Rev. B 87, 205125, 2013). All pseudopotentials arefrom the Quantum ESPRESSO library for reproducibility. The planewaveskinetic energy cutoffs for the wavefunctions and charge density are setto 60/70 Ry and 300/350 Ry for Bi/Sb perovskites, respectively. TheBrillouin zone is sampled using an unshifted 6×6×6 grid. Forces andtotal energies are converged to 10 meV/A and 1 meV. All structuraloptimisations were carried out using scalar-relativistic LDA, while theband structures were from fully-relativistic calculations. Theeigenvalues of high-symmetry points were also calculated using the PBEOfunctional in order to approximately correct for the band gapunderestimation in DFT/LDA. This choice is potentially of limitedaccuracy but is expected to provide reliable band gap variations withrespect to MAPbI₃. The PBEO calculations are carried out using VASP, theprojector-augmented wave method, and a kinetic energy cut-off of 37 Ry.In the shake-and-relax method we displace every atomic coordinaterandomly by ±0.1 Å, and randomly modify each component of the directlattice vectors by ±0.3 Å. For each structure repeat this procedure isrepeated starting from 10 randomised configurations. As a cross-test wechecked that MAPb₃ correctly passes this test, while LiPbI₃ (which isknow not to form in a perovskite structure) does not. The carriereffective masses are calculated using the relativistic DFT/LDA bandstructures, using a second-order finite-differences formula withincrements of 4×10⁻³ Å⁻¹ in reciprocal space. The anisotropy ratio ofthe effective mass tensor is obtained as the norm of the traceless partof the tensor over the norm of the complete tensor (this ratio can bebetween 0 and 100%). The optical absorption spectrum is obtained asα(ω)=ωϵ₂(ω)/cn(ω), where ω is the photon frequency, c the speed oflight, ϵ₂ the imaginary part of the dielectric function, and n therefractive index. The dielectric function is calculated within theindependent particle approximation, by taking the average over the lightpolarization vectors, using a dense 8×8×8 Brillouin zone grid. In thecalculations of optical spectra the DFT/LDA band structures are usedafter applying a scissor correction to the conduction states in order tomatch the PBEO band gaps.

Results of Computational Study

Hypothetical double perovskites derived from MAPbI₃ by replacing two Pbatoms with the pairs B^(I)/B^(III) where B^(III)═Sb or Bi and B^(I)═Cu,Ag or Au were investigated. In particular, it was investigated what theoptoelectronic properties of such double perovskites would be whenadopting the same structure as MAPbI₃. Starting from the low-temperatureorthorhombic structure of MAPbI₃, Pb was substituted for B^(I)/B^(III)using a rock-salt ordering with B^(I) and B^(III) alternating in everydirection. For each structure thus constructed standard structuraloptimization is performed using density-functional theory (DFT) in thelocal density approximation (LDA), including relativistic spin-orbitcoupling effects throughout. In the band structure calculations the PBEOfunctional was employed in order to bracket the band gaps.

In the case of Bi-based hypothetical double perovskites, MA₂AgBiI₆ wasas focussed on as a representative example. FIG. 2 shows that the bandstructure of MA₂AgBiI₆ exhibits strongly dispersive band edges, with thevalence and conduction band extrema at F (wavevector k=0 in theBrillouin zone) of I-5p and of mixed Bi-6p and I-5p character,respectively. The dispersive profile and the p-character of the bandedges at Γ bear a close resemblance to the electronic structure ofMAPbI₃. The band gap at Γ (0.7/2.1 eV within LDA/PBEO) is also verysimilar to the value calculated for MAPbI₃ (0.4/1.7 eV; the experimentalvalue is 1.7 eV). However, unlike in MAPbI₃, the top of the valence bandis now at the S point [k=(π/a; π/b; 0)], and the bottom of theconduction band is at the T point [k=(0; π/b; π/c)], where a=8.51 Å andb=8.04 Å and c=11.91 Å are the lattice parameters. By comparing the bandstructures of MAPbI₃ and MA₂AgBiI₆ in FIGS. 1 and 2 it is seen that theemergence of an indirect gap in the latter results from the folding ofthe Brillouin zone along the directions of alternation of Bi and Ag,whereby Γ is mapped into the T and S points. As a result MA₂AgBiI₆exhibits an indirect band gap only slightly below (0.15 eV) the directgap at Γ. This unusual band structure could prove beneficial to solarcell performance, since the absorption onset and the maximumopen-circuit voltage are similar to the case of MAPbI₃, but unlikeMAPbI₃ the radiative recombination of photo-excited carriers isquenched. FIGS. 3 and 4 shows that the phenomenology just described forMA₂AgBiI₆ remains essentially unchanged for the Cu and Au perovskitesMA₂CuBiI₆ and MA₂AuBiI₆, therefore each of these compounds exhibits anelectronic structure which is promising for replacing lead in MAPbI₃.

A formamidinium (FA) analogue of the double perovskite compoundsdiscussed above was also investigated. This compound, FA₂AgBiI₆, has afundamental band gap of 0.2/1.5 eV (LDA/PBEO) as shown in FIG. 5, whichis 0.3 eV smaller than that found for MA₂AgBiI₆. The band gap ofFA₂AgBiI₆ is remarkably close to the value calculated for MAPbI₃. Basedon these results it is proposed that FA₂AgBiI₆ is also a promisingcandidate to achieve lead-free perovskite solar cells.

The electronic structure of a hypothetical MABi_(0.5)Na_(0.5)I₃ doubleperovskites was also calculated and is shown in FIG. 6. The structure ofthe compound is obtained from MAPbI₃ by replacing Pb with Bi and Na,arranged in the rock-salt ordering, and fully optimising the resultingstructure within DFT/LDA. The band structure is reported along the samehigh-symmetry lines as in FIG. 1. The fundamental gap calculated inDFT/LDA is 1.6 eV. The hole effective mass along the Γ-X and Γ-Ydirection is 9.5 m_(e) and 5.6 m_(e), respectively.

Thus, based on these computational studies, a range of halide doubleperovskite compounds are shown to be promising alternatives tolead-based perovskites.

Example 2—Synthesis of MA₂AgBi₆

The organic-inorganic halide double perovskite methylammonium bismuthsilver iodide (MA₂AgBi₆) was synthesised by the following procedure.

(CH₃NH₃)I (methylammonium iodide, MAI), BiI₃ and AgI were independentlydissolved in dimethylformamide (DMF). In particular, 0.250 g of AgI,0.628 g of BiI₃ and 0.338 g of MAI were each dissolved in a separatevial (a molar ratio of AgI:BiI₃:MAI of 1:1:2 was used). To completelydissolve BiI₃ and AgI in DMF the solutions in different vials wereheated on a hot plate up to ˜100 degrees centigrade while stirring. Thethree solutions of AgI, BiI₃ and MAI were mixed in a 15 ml vial. Themixed solution was left on the hot plate at 115 degrees centigrade for15 minutes before it was filtered. This was deposited on a fluorinedoped tin oxide (FTO) coated glass by spin coating at 2000 rpm for 30seconds. After drying in air the films were cured at 120 degreescentigrade to remove the solvent for 20 minutes and subsequently cooledto room temperature. Smooth films were obtained.

A powder x-ray diffraction pattern of the resulting solid MA₂AgBiI₆ wasrecorded on an X'Pert PRO from PANalytical equipped with a Cu X-ray tube(K_(α1)=1.54060 Å), a secondary graphite (002) monochromator and aposition sensitive X'Celerator detector, and operated in Bragg-Brentanogeometry. The results are shown in FIG. 7. The compound was found tohave a monoclinic crystal structure with the cell parameters: a=13.2284Å; b=6.3402 Å; c=11.5789 Å; α=90.0000°; β=112.6570°; γ=90.0000°.

A UV-Vis spectrum for MA₂AgBiI₆ was also carried out using a Cary 300UV-visible spectrophotometer in a photometric range of 200-850 nm andthe result is shown in FIG. 9.

Example 3—Synthesis of FA₂AgBiI₆

The organic-inorganic halide double perovskite formamidinium bismuthsilver iodide (FA₂AgBiI₆) was synthesised by the following procedure.

(H₂N—C(H)═NH₂)I (formamidinium iodide, FAI), Bi₃ and AgI wereindependently dissolved in dimethylformamide (DMF). In particular, 0.250g of AgI, 0.628 g of BiI₃ and 0.366 g of FAI were each dissolved in aseparate vial (a molar ratio of AgI:BiI₃: FAI 1:1:2 was used). Tocompletely dissolve BiI₃ and AgI in DMF the solutions in different vialswere heated on a hot plate up to ˜100 degrees centigrade while stirring.A reddish solution formed upon mixing the three solutions of AgI, BiI₃and FAI in a 15 ml vial. The mixed solution was left on the hot plate at115 degrees centigrade for 15 minutes before it was filtered. This wasdeposited on a fluorine doped tin oxide (FTO) coated glass by spincoating at 2000 rpm for 30 seconds. After drying in air the films werecured at 120 degrees centigrade to remove the solvent for 20 minutes andsubsequently cooled to room temperature. Smooth films with orangereddish colour were obtained.

A powder x-ray diffraction pattern of the resulting solid FA₂AgBiI₆ wasrecorded on an X'Pert PRO from PANalytical equipped with a Cu X-ray tube(K_(α1)=1.54060 Å), a secondary graphite (002) monochromator and aposition sensitive X'Celerator detector, and operated in Bragg-Brentanogeometry. The results are shown in FIG. 8. The compound was found tohave a cubic crystal structure with the cell parameters: a=b=c=10.781 Å;α=β=γ=90.0000°.

A UV-Vis spectrum for FA₂AgBiI₆ was also carried out using a Cary 300UV-visible spectrophotometer in a photometric range of 200-850 nm andthe result is shown in FIG. 10.

Example 4—Synthesis of MA₂AuBiI₆

The organic-inorganic halide double perovskite formamidinium bismuthgold iodide (MA₂AuBiI₆) was synthesised by the following procedure.

2 equivalent of methylammonium iodide (CH₃NH₃I, MAI), 1 equivalent ofgold iodide (AuI) and 1 equivalent of bismuth triiodide (BiI₃) weredissolved in dimethylformamide (DMF). The resulting solution wasspin-coated on to glass and the film was dried to produce a layer ofMA₂AuBiI₆.

A UV-Vis spectrum for MA₂AuBiI₆ was also carried out using a Cary 300UV-visible spectrophotometer in a photometric range of 200-850 nm andthe result is shown in FIG. 11.

Example 5—Photoluminescence Measurements

Measurements of the photoluminescence emission spectra of MA₂AgBiI₆ andFA₂AgBiI₆ were done using a Fluorolog spectrometer with excitationwavelength of 450 nm and measurements from 470 nm-800 nm with anintegration time of 0.4 seconds. The results are shown in FIG. 12.

Example 6—Synthesis and Characterisation of Cs₂BiAgCl₆

Synthesis: Single-phase samples of Cs₂BiAgCl₆ were prepared bysolid-state reaction in a sealed fused silica ampoule. For a typicalreaction, the starting materials CsCl (Sigma Aldrich, 99.9%), BiCl₃(Sigma Aldrich, 99.99%) and AgCl (Sigma Aldrich, 99%) were mixed at amolar ratio of 2:1:1 respectively. The mixture was loaded in a fusedsilica ampoule that was flame sealed under vacuum (10⁻³ Torr). Themixture was heated to 500° C. over 5 hours and held at 500° C. for 4hours. After cooling to room temperature, a yellow polycrystallinematerial was formed. Octahedral shaped crystals of length ˜0.1 mm couldbe extracted from the powder sample, and these octahedral crystals wereused to determine the crystal structure of the compound.

Film Fabrication:

Cs₂BiAgCl₆ powder was dispersed in poly methyl methacrylate (PMMA) inToluene. To form films, the dispersion was spin-coated on a glass slideat 1500 rpm. This was repeated several times to attain a uniform thickfilm.

Structural Characterization:

Powder X-ray diffraction was carried out using a Panalytical X'pertpowder diffractometer (Cu-Kα1 radiation; λ=154.05 pm) at roomtemperature. Structural parameters were obtained by Rietveld refinementusing General Structural Analysis Software. Single crystal data werecollected for Cs₂BiAgCl₆ at room temperature using an Agilent Supernovadiffractometer that uses Mo Kα beam with λ=71.073 pm and is fitted withan Atlas detector. Data integration and cell refinement were performedusing CrysAlis Pro Software by Agilent Technogies Ltd, Yarnton,Oxfordshire, England. The structure was analysed by Patterson and Directmethods and refined using the SHELXL 2014 software package.

Film Characterization:

A Varian Cary 300 UV-Vis spectrophotometer with an integrating spherewas used to acquire absorbance spectra and to account for reflection andscattering. Time-resolved photo-luminescence measurements were acquiredusing a time correlated single photon counting (TCSPC) setup (FluoTime300, PicoQuant GmbH). Film samples were photoexcited using a 397 nmlaser head (LDH P-C-405, PicoQuant GmbH) pulsed at frequencies of 200kHz. The steady-state photoluminescence (PL) measurements were takenusing an automated spectrflouorometer (Fluorolog, Horiba Jobin-Yvon),with a 450 W-Xenon lamp excitation.

The results of the single crystal analysis are shown in Table 1 below.FIG. 13 shows characterisation results for Cs₂BiAgCl₆. FIG. 13 (a) showsX-ray diffraction pattern for a Cs₂BiAgCl₆ single crystal at 293 K. hklis shown for three different planes, i.e. 0kl, h0l and hk0. All wavevectors are labelled in reciprocal lattice units (rlu) and a*, b* and c*denote reciprocal lattice vectors of the cubic cell of the Fm3mstructure. FIG. 13 (b) shows the UV-Vis optical absorption spectrum ofCs₂BiAgCl₆. The inset shows the Tauc plot, corresponding to an indirectallowed transition. The straight lines are fitted to the linear regionsof the absorption spectrum and Tauc plot, and the intercepts at 2.32 eVand 2.54 eV marked on the plot are calculated from the fit. FIG. 13 (c)shows the steady-state photoluminescence (PL) spectrum of Cs₂BiAgCl₆deposited on glass. Finally, FIG. 13 (d) shows the time resolvedphotoluminescence decay of Cs₂BiAgCl₆ deposited on glass. The data isfitted using a biexponential decay function. The decay lifetimes of 15ns (fast) and 100 ns (slow) is estimated from the fit.

The X-ray Diffraction Pattern in FIG. 13 (a) is shown for a singlecrystal (˜30 μm diameter). Sharp reflections are observed for thecrystallographic 0kl, h0l and hk0 planes. These reflections showcharacteristics of m3m symmetry that reveal systematic absences for(hkl; h+k, k+l, h+l=2n) corresponding to the face-centered space groupsF432, F43m and Fm3m. The latter was selected for structure refinementafter confirmation that Cs₂BiAgCl₆ crystallizes in an FCC lattice. Itwas found that there is no significant distortion of octahedral symmetryabout the Bi³⁺. The X-ray diffraction patterns uniquely identify theFm3m (no. 225) space group at room temperature, and the quantitativestructural analysis gives a very good description of the data. Inaddition, the crystal structure refinement is consistent with therock-salt configuration assumed by the atomistic model. The experimentaland computationally predicted conventional lattice parameters are invery good agreement, 10.78 Å and 10.50 Å, respectively. From the opticalabsorption spectrum and Tauc plot (see FIG. 13 (b)) an indirect opticalband gap in the range of 2.3-2.5 eV was estimated. The indirectcharacter of the band gap is consistent with the broad photoluminescencepeak observed between 480 and 650 nm (1.9-2.6 eV) with the maximum at˜575 nm (2.2 eV), red-shifted with respect to the optical absorptiononset. In addition, the time-resolved photoluminescence decay shown inFIG. 13 (c) was fitted with a double exponential giving a fast componentlifetime of 15 ns and a slow component lifetime of 100 ns.

TABLE 1 single crystal data for Cs₂BiAgCl₆ Compound Cs₂BiAgCl₆Measurement temperature 293 K Crystal system Cubic Space group Fm3m Unitcell dimensions a = 10.777 ± 0.005 Å α = β = γ = 90° Volume 1251.68 Å³ Z4 Density (calculated) 4.221 g/cm³ Reflections collected 3434 Uniquereflections 82 from which 0 suppressed R(int) 0.1109 R (sigma) 0.0266Goodness-of-fit 1.151 Final R indices (R_(all)) 0.0212 wR_(obs) 0.0322Wavelength 0.71073 Å

The electronic band structure of Cs₂BiAgCl₆ was calculated based on theexperimentally determined crystal structure, with and withoutrelativistic spin-orbit coupling (SOC) effects. The results are shown inFIG. 14. The features of the valence band edge are almost unchanged whenthe relativistic effects are included. This is consistent with thepredominant Cl-p and Ag-d character of this band. By contrast, due tothe large spin-orbit coupling, the conduction band edge splits in twobands, separated by more than 1.5 eV at the Γ point. This effect is notsurprising, given that the character of the conduction band bottom is ofprimarily Bi-p character. The fundamental band gap is reduced by 0.4 eVupon inclusion of relativistic effects, and the shape of the conductionband is drastically different. In the fully relativistic case wecalculated an indirect band gap of 3.0 eV and lowest direct transitionof 3.5 eV. The calculated electronic band gaps are overestimated withrespect to the measured optical band gap by approximately 0.5 eV. Thisquantitative discrepancy does not affect the qualitative physical trendsof the band gaps.

In the Example, Cs₂BiAgCl₆ was successfully synthesised and found tohave a face-centred cubic double perovskite structure. The compound wasfound to exhibit optical properties consistent with an indirect gapsemiconductor. This was in agreement with the computational study.

Example 7—Synthesis and Characterisation of Cs₂BiAgBr₆

Solution-Based Synthesis and Crystal Growth of Cs₂BiAgBr₆

Samples of Cs₂BiAgBr₆ were prepared by precipitation from an acidicsolution of hyrobromic acid. A mixture of a 1 mmol BiBr₃ (Sigma Aldrich,99.99%) and AgBr (Sigma Aldrich, 99%) were first dissolved in 12 ml 8.84M HBr. 2 mmol of CsBr (Sigma Aldrich, 99.9%) were added and the solutionwas heated to 150° C. to dissolve the salts. The solution was cooled to118° C. at 4° C./hour to initiate supersaturation and produce singlecrystals.

High-purity polycrystalline samples were synthesised by the followingmethod. A mixture of 8 ml (8.84 M) HBr and 2 ml 50 wt % H₃PO₂ solutionwas heated to 120° C. and 1.31 mmol of AgBr and BiBr₃ dissolved into it.Adding 2.82 mmol of CsBr caused an orange precipitate to formimmediately. The hot solution was left for 30 minutes under gentlestirring to ensure a complete reaction before being filtered and theresulting solid washed with ethanol and dried in a furnace.

Synthesis Via Solid-State Reaction

Single-phase samples of Cs₂BiAgBr₆ were prepared by conventionalsolid-state reaction in a sealed fused silica ampoule. For a typicalreaction, the starting materials CsBr (Sigma Aldrich, 99.9%), BiBr₃(Sigma Aldrich, 99.99%) and AgBr (Sigma Aldrich, 99%) were mixed in amolar ratio 2:1:1, respectively. The mixture was loaded in a fusedsilica ampoule that was flame sealed under vacuum (10⁻³ Torr). Themixture was heated to 500° C. over 5 hours and held at 500′C for 4hours. After cooling to room temperature, an orange polycrystallinematerial was formed which was Cs₂BiAgBr₆. Octahedral shaped crystals ofmaximum size 1 mm³ could be extracted from the powder samples that laterwere used to determine the crystal structures.

Structural Characterization

Powder X-ray diffraction was carried out using a Panalytical X'pertpowder diffractometer (Cu-Kα1 radiation; λ=154.05 pm) at roomtemperature. The XRPD pattern for Cs₂BiAgBr₆ is shown in FIG. 15.Structural parameters were obtained by Rietveld refinement using GeneralStructural Analysis Software. Single crystal data were collected forCs₂BiAgBr₆ at room temperature using an Agilent Supernova diffractometerthat uses Mo Kα beam with λ=71.073 pm and is fitted with an Atlasdetector. Data integration and cell refinement was performed usingCrysAlis Pro Software (Agilent Technologies Ltd., Yarnton, Oxfordshire,England). The structure was analysed by Patterson and Direct methods andrefined using SHELXL 2014 software package. The single crystal data isshown in Table 2.

TABLE 2 single crystal data for Cs₂BiAgBr₆ Compound Cs₂BiAgBr₆Measurement temperature 293 K Crystal system Cubic Space group F m −3 mUnit cell dimensions a = 11.264 ± 0.005 Å, α = β = γ = 90° Volume1429.15 Å Z 4 Density (calculated) 4.936 g/cm³ Reflections collected3830 Unique reflections 95 from which 0 suppressed R(int) 0.0691 R(sigma) 0.0150 Goodness-of-fit 0.369 Final R indices (R_(all)) 0.0192wR_(obs) 0.0676 Wavelength 0.71073 Å

Optical Characterization

A Varian Cary 300 UV-Vis spectrophotometer with an integrating spherewas used to acquire absorbance spectra and to account for reflection andscattering. The room temperature optical absorption spectra ofCs₂BiAgBr₆ is shown in FIG. 16.

A 397.7 nm laser diode (Pico-Quant LDH P-C-405) was used forphotoexcitation and pulsed at frequencies ranging from 1-80 MHz. Thesteady-state photoluminescence (PL) measurements were taken using anautomated spectrofluorometer (Fluorolog, Horiba Jobin-Yvon), with a 450W-Xenon lamp excitation. FIG. 17 shows the room temperaturephotoluminescence spectrum measured for Cs₂BiAgBr₆.

1. A conductor device comprising a semiconducting material, wherein thesemiconducting material comprises a compound comprising: (i) one or morefirst monocations [A]; (ii) one or more second monocations [B^(I)];(iii) one or more trications [B^(III)]; and (iv) one or more halideanions [X]. 2-38. (canceled)
 39. A semiconductor device according toclaim 1, wherein the one or more first monocations [A] are selected frommetal monocations and organic monocations.
 40. A semiconductor deviceaccording to claim 1, wherein the one or more first monocations [A] areselected from K⁺, Rb⁺, Cs⁺, (NR¹R²R³R⁴)⁺, (R¹R²N═CR³R⁴)⁺,(R¹R²N—C(R⁵)═NR³R⁴)⁺ and (R¹R²N—C(NR⁵R⁶)═NR³R⁴)⁺, wherein each of R¹,R², R³, R⁴, R⁵ and R⁶ is independently H, a substituted or unsubstitutedC₁₋₂₀ alkyl group or a substituted or unsubstituted aryl group.
 41. Asemiconductor device according to claim 1, wherein the one or moresecond monocations [B^(I)] are selected from metal and metalloidmonocations.
 42. A semiconductor device according to claim 1, whereinthe one or more second monocations [B^(I)] are selected from Li⁺, Na⁺,K⁺, Rb⁺, Cs⁺, Cu⁺, Ag⁺, Au⁺ and Hg⁺.
 46. A semiconductor deviceaccording to claim 1, wherein the one or more trications [B^(III)] areselected from metal and metalloid trications.
 44. A semiconductor deviceaccording to claim 1, wherein the one or more trications [B^(III)] areselected from Bi³⁺, Sb³⁺, Cr³⁺, Fe³⁺, Co³⁺, Ga³⁺, As³⁺, Ru³⁺, Rh³⁺,In³⁺, Ir³⁺ and Au³⁺.
 45. A semiconductor device according to claim 1,wherein the one or more halide anions [X] are selected from I⁻, Br⁻, Cl⁻and F⁻.
 46. A semiconductor device according to claim 1, wherein thecompound is a double perovskite compound of formula (I):[A]₂[B^(I)][B^(III)][X]₆  (I); wherein: [A] is the one or more firstmonocations; [B^(I)] is the one or more second monocations; [B^(III)] isthe one or more trications; and [X] is the one or more halide anions.47. A semiconductor device according to claim 1, wherein the compound isa double perovskite compound of formula (Ia):A₂B^(I)B^(III)[X]₆  (Ia); wherein: A is one first monocation; B^(I) isone second monocation; B^(III) is one trication; and [X] is the one ormore halide anions.
 48. A semiconductor device according to claim 1,wherein the compound is (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂CuBiI₆, (H₂N—C(H)═NH₂)₂AgSbI₆, (H₂N—C(H)═NH₂)₂AuSbI₆,(H₂N—C(H)═NH₂)₂CuSbI₆, (H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆,(H₂N—C(H)═NH₂)₂CuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(H₂N—C(H)═NH₂)₂CuSbBr₆, (H₂N—C(H)═NH₂)₂AgBiCl₆, (H₂N—C(H)═NH₂)₂AuBiCl₆,(H₂N—C(H)═NH₂)₂CuBiCl₆, (H₂N—C(H)═NH₂)₂AgSbCl₆, (H₂N—C(H)═NH₂)₂AuSbCl₆,(H₂N—C(H)═NH₂)₂CuSbCl₆, (H₂N—C(H)═NH₂)₂AgBiF₆, (H₂N—C(H)═NH₂)₂AuBiF₆,(H₂N—C(H)═NH₂)₂CuBiF₆, (H₂N—C(H)═NH₂)₂AgSbF₆, (H₂N—C(H)═NH₂)₂AuSbF₆,(H₂N—C(H)═NH₂)₂CuSbF₆, (CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆,(CH₃NH₃)₂CuBiI₆, (CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂CuSbI₆,(CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆, (CH₃NH₃)₂CuBiBr₆, (CH₃NH₃)₂AgSbBr₆,(CH₃NH₃)₂AuSbBr₆, (CH₃NH₃)₂CuSbBr₆, (CH₃NH₃)₂AgBiCl₆, (CH₃NH₃)₂AuBiCl₆,(CH₃NH₃)₂CuBiCl₆, (CH₃NH₃)₂AgSbCl₆, (CH₃NH₃)₂AuSbCl₆, (CH₃NH₃)₂CuSbCl₆,(CH₃NH₃)₂AgBiF₆, (CH₃NH₃)₂AuBiF₆, (CH₃NH₃)₂CuBiF₆, (CH₃NH₃)₂AgSbF₆,(CH₃NH₃)₂AuSbF₆, (CH₃NH₃)₂CuSbF₆, Cs₂AgBiI₆, Cs₂AuBiI₆, Cs₂CuBiI₆,Cs₂AgSbI₆, Cs₂AuSbI₆, Cs₂CuSbI₆, Cs₂AgBiBr₆, Cs₂AuBiBr₆, Cs₂CuBiBr₆,Cs₂AgSbBr₆, Cs₂AuSbBr₆, Cs₂CuSbBr₆, Cs₂AgBiCl₆, Cs₂AuBiCl₆, Cs₂CuBiCl₆,Cs₂AgSbCl₆, Cs₂AuSbCl₆, Cs₂CuSbI₆, Cs₂AgBiF₆, Cs₂AuBiF₆, Cs₂CuBiF₆,Cs₂AgSbF₆, Cs₂AuSbF₆ or Cs₂CuSbF₆.
 49. A semiconductor device accordingto claim 1, wherein the compound is (H₂N—C(H)═NH₂)₂AgBiI₆,(H₂N—C(H)═NH₂)₂AuBiI₆, (H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆,(H₂N—C(H)═NH₂)₂AgSbI₆, (H₂N—C(H)═NH₂)₂AuSbI₆, (H₂N—C(H)═NH₂)₂AgSbBr₆,(H₂N—C(H)═NH₂)₂AuSbBr₆, (CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆,(CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆, (CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆,(CH₃NH₃)₂AgSbBr₆ or (CH₃NH₃)₂AuSbBr₆.
 50. A semiconductor deviceaccording to claim 1, wherein the compound is Cs₂AgBiCl₆.
 51. Asemiconductor device according to claim 1, wherein the compound is alayered double perovskite compound of formula (II):[A]₄[B^(I)][B^(III)][X]₈  (II); wherein: [A] is the one or more firstmonocations; [B^(I)] is the one or more second monocations; [B^(III)] isthe one or more trications; and [X] is the one or more halide anions.52. A semiconductor device as defined in claim 51, wherein the compoundis (R¹NH₃)₄AgBiI₈, (R¹NH₃)₄AuBiI₈, (R¹NH₃)₄CuBiI₈, (R¹NH₃)₄AgSbI₈,(R¹NH₃)₄AuSbI₈, (R¹NH₃)₄CuSbI₈, (R¹NH₃)₄AgBiBr₈, (R¹NH₃)₄AuBiBr₈,(R¹NH₃)₄CuBiBr₈, (R¹NH₃)₄AgSbBr₈, (R¹NH₃)₄AuSbBr₈, (R¹NH₃)₄CuSbBr₈,(R¹NH₃)₄AgBiCl₈, (R¹NH₃)₄AuBiCl₈, (R¹NH₃)₄CuBiCl₈, (R¹NH₃)₄AgSbCl₈,(R¹NH₃)₄AuSbCl₈, (R¹NH₃)₄CuSbCl₈, (R¹NH₃)₄AgBiF₈, (R¹NH₃)₄AuBiF₈,(R¹NH₃)₄CuBiF₈, (R¹NH₃)₄AgSbF₈, (R¹NH₃)₄AuSbF₈ or (R¹NH₃)₄CuSbF₈,wherein R¹ is an unsubstituted C₃₋₁₂ alkyl group.
 53. A semiconductordevice according to claim 1, wherein the compound further comprises oneor more metal or metalloid dications.
 54. A semiconductor deviceaccording to claim 53, wherein the compound is a hybrid doubleperovskite compound of formula (Iz) or a layered hybrid doubleperovskite compound of formula (IIz):[A]₂[B^(I)]_((2-x)/2)[B^(II)]_(x)[B^(III)]_((2-x)/2)[X]₆  (Iz);[A]₄[B^(I)]_((2-x)/2)[B^(II)]_(x)[B^(III)]_((2-x)/2)[X]⁸  (IIz); wherein[A] is the one or more first monocations; [B^(I)] is the one or moresecond monocations; [B^(II)] is the one or more metal or metalloiddications; [B^(III)] is the one or more trications; [X] is the one ormore halide anions; and x is from 0.0 to 1.98.
 55. A semiconductordevice according to claim 53, wherein the compound is a doubleperovskite compound of formula (Iza) or a layered double perovskitecompound of formula (IIza):[A]₂[B^(I)]_(0.5)Pb[B^(III)]_(0.5)[X]₆  (Iza);[A]₄[B^(I)]_(0.5)Pb[B^(III)]_(0.5)[X]₈  (IIza); wherein [A] is the oneor more first monocations; [B^(I)] is the one or more secondmonocations; [B^(III)] is the one or more trications; and [X] is the oneor more halide anions.
 56. A semiconductor device according to claim 1,wherein the semiconductor device is a photovoltaic device.
 57. Asemiconductor device according to claim 1, which semiconductor devicecomprises a layer of the semiconducting material, which layer preferablyhas a thickness of from 5 nm to 1000 nm.
 58. A semiconductor deviceaccording to claim 1, which semiconductor device comprises: an n-typeregion comprising at least one n-type layer; a p-type region comprisingat least one p-type layer; and, disposed between the n-type region andthe p-type region: a layer of the semiconducting material.
 59. Asemiconductor device according to claim 1, which semiconductor devicecomprises a layer of said semiconducting material without open porosity.60. A semiconductor device according to claim 58, wherein the layer ofthe semiconducting material forms a planar heterojunction with then-type region or the p-type region, or wherein the layer of thesemiconducting material forms a first planar heterojunction with then-type region and a second planar heterojunction with the p-type region.61. A semiconductor device according to claim 1, wherein thesemiconductor device comprises: an n-type region comprising at least onen-type layer; a p-type region comprising at least one p-type layer; and,disposed between the n-type region and the p-type region: (i) a porousscaffold material; and (ii) said semiconducting material in contact withthe scaffold material.
 62. A semiconductor device according to claim 58,wherein the n-type region comprises a compact layer of an n-typesemiconductor, or wherein the p-type region comprises a layer of anorganic p-type semiconductor.
 63. A semiconductor device according toclaim 1, wherein said semiconducting material is a photoactive material.64. A process for producing a semiconductor device comprising asemiconducting material, wherein the semiconducting material comprises acompound comprising: (i) one or more first monocations [A]; (ii) one ormore second monocations [B^(I)]; (iii) one or more trications [B^(III)];and (iv) one or more halide anions [X], which process comprises: (a)disposing a second region on a first region, which second regioncomprises a layer of said semiconducting material.
 65. A processaccording to claim 64, wherein the process further comprises (b)disposing a third region on the second region, wherein: said firstregion is an n-type region comprising at least one n-type layer and saidthird region is a p-type region comprising at least one p-type layer; orsaid first region is a p-type region comprising at least one p-typelayer and said third region is an n-type region comprising at least onen-type layer.
 66. A process according to claim 64, wherein (a) disposinga second region on a first region comprises: (Ai) exposing the firstregion to vapour, which vapour comprises said semiconducting material orone or more reactants for producing said semiconducting material; and(Aii) allowing deposition of the vapour onto the first region to producea layer of said semiconducting material thereon; or (Bi) disposing oneor more precursor compositions on the first region, which one or moreprecursor compositions comprise: said semiconducting material and one ormore solvents; or one or more reactants for producing saidsemiconducting material and one or more solvents; and (Bii) removing theone or more solvents to produce on the first region a layer of saidsemiconducting material.
 67. A process according to claim 64, whereinthe one or more reactants for producing the semiconducting materialcomprise one or more first precursor compounds, one or more secondprecursor compounds and one or more third precursor compounds, which oneor more first precursor compounds are selected from compounds of formula[A][X]; which one or more second precursor compounds are selected fromcompounds of formula [B^(I)][X]; and which one or more second precursorcompounds are selected from compounds of formula [B^(III)][X]₃; wherein[A] is the one or more first monocations; [B^(I)] is the one or moresecond monocations; [B^(III)] is the one or more trications; and each[X] is the one or more halide anions.
 68. A compound comprising: (i) oneor more first monocations [A]; (ii) one or more second monocations[B^(I)] selected from Cu⁺, Ag⁺ and Au⁺; (iii) one or more trications[B^(III)]; and (iv) one or more halide anions [X].
 69. A compoundaccording to claim 68, wherein the compound is a double perovskitecompound of formula (I) or a layered double perovskite compound offormula (II):[A]₂[B^(I)][B^(III)][X]₆  (I);[A]₄[B^(I)][B^(III)][X]₈  (II); wherein [A] is the one or more firstmonocations; [B^(I)] is the one or more second monocations selected fromCu⁺, Ag⁺ and Au⁺; [B^(III)] is the one or more trications; and [X] isthe one or more halide anions.
 70. A compound according to claim 68,wherein the compound is (H₂N—C(H)═NH₂)₂AgBiI₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂CuBiI₆, (H₂N—C(H)═NH₂)₂AgSbI₆, (H₂N—C(H)═NH₂)₂AuSbI₆,(H₂N—C(H)═NH₂)₂CuSbI₆, (H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆,(H₂N—C(H)═NH₂)₂CuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(H₂N—C(H)═NH₂)₂CuSbBr₆, (H₂N—C(H)═NH₂)₂AgBiCl₆, (H₂N—C(H)═NH₂)₂AuBiCl₆,(H₂N—C(H)═NH₂)₂CuBiCl₆, (H₂N—C(H)═NH₂)₂AgSbCl₆, (H₂N—C(H)═NH₂)₂AuSbCl₆,(H₂N—C(H)═NH₂)₂CuSbCl₆, (H₂N—C(H)═NH₂)₂AgBiF₆, (H₂N—C(H)═NH₂)₂AuBiF₆,(H₂N—C(H)═NH₂)₂CuBiF₆, (H₂N—C(H)═NH₂)₂AgSbF₆, (H₂N—C(H)═NH₂)₂AuSbF₆,(H₂N—C(H)═NH₂)₂CuSbF₆, (CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆,(CH₃NH₃)₂CuBiI₆, (CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂CuSbI₆,(CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆, (CH₃NH₃)₂CuBiBr₆, (CH₃NH₃)₂AgSbBr₆,(CH₃NH₃)₂AuSbBr₆, (CH₃NH₃)₂CuSbBr₆, (CH₃NH₃)₂AgBiCl₆, (CH₃NH₃)₂AuBiCl₆,(CH₃NH₃)₂CuBiCl₆, (CH₃NH₃)₂AgSbCl₆, (CH₃NH₃)₂AuSbCl₆, (CH₃NH₃)₂CuSbCl₆,(CH₃NH₃)₂AgBiF₆, (CH₃NH₃)₂AuBiF₆, (CH₃NH₃)₂CuBiF₆, (CH₃NH₃)₂AgSbF₆,(CH₃NH₃)₂AuSbF₆, (CH₃NH₃)₂CuSbF₆, Cs₂AgBiI₆, Cs₂AuBiI₆, Cs₂CuBiI₆,Cs₂AgSbI₆, Cs₂AuSbI₆, Cs₂CuSbI₆, Cs₂AgBiBr₆, Cs₂AuBiBr₆, Cs₂CuBiBr₆,Cs₂AgSbBr₆, Cs₂AuSbBr₆, Cs₂CuSbBr₆, Cs₂AgBiCl₆, Cs₂AuBiCl₆, Cs₂CuBiCl₆,Cs₂AgSbCl₆, Cs₂AuSbCl₆, Cs₂CuSbCl₆, Cs₂AgBiF₆, Cs₂AuBiF₆, Cs₂CuBiF₆,Cs₂AgSbF₆, Cs₂AuSbF₆ or Cs₂CuSbF₆.
 71. A compound according to claim 68,wherein the compound is (H₂N—C(H)═NH₂)₂AgBi₆, (H₂N—C(H)═NH₂)₂AuBiI₆,(H₂N—C(H)═NH₂)₂AgBiBr₆, (H₂N—C(H)═NH₂)₂AuBiBr₆, (H₂N—C(H)═NH₂)₂AgSbI₆,(H₂N—C(H)═NH₂)₂AuSbI₆, (H₂N—C(H)═NH₂)₂AgSbBr₆, (H₂N—C(H)═NH₂)₂AuSbBr₆,(CH₃NH₃)₂AgBiI₆, (CH₃NH₃)₂AuBiI₆, (CH₃NH₃)₂AgBiBr₆, (CH₃NH₃)₂AuBiBr₆,(CH₃NH₃)₂AgSbI₆, (CH₃NH₃)₂AuSbI₆, (CH₃NH₃)₂AgSbBr₆ or (CH₃NH₃)₂AuSbBr₆.72. A compound according to claim 68, wherein the compound isCs₂AgBiCl₆.